GNU 'troff'
1 Introduction
  1.1 Background
  1.2 What Is 'groff'?
  1.3 'groff' Capabilities
  1.4 Macro Packages
  1.5 Preprocessors
  1.6 Output Devices
  1.7 Installation
  1.8 Conventions Used in This Manual
  1.9 Credits
2 Invoking 'groff'
  2.1 Options
  2.2 Environment
  2.3 Macro Directories
  2.4 Font Directories
  2.5 Paper Format
  2.6 Invocation Examples
3 Tutorial for Macro Users
  3.1 Basics
  3.2 Common Features
    3.2.1 Paragraphs
    3.2.2 Sections and Chapters
    3.2.3 Headers and Footers
    3.2.4 Page Layout
    3.2.5 Displays
    3.2.6 Footnotes and Annotations
    3.2.7 Table of Contents
    3.2.8 Indices
    3.2.9 Paper Formats
    3.2.10 Multiple Columns
    3.2.11 Font and Size Changes
    3.2.12 Predefined Strings
    3.2.13 Preprocessor Support
    3.2.14 Configuration and Customization
4 Macro Packages
  4.1 'man'
    4.1.1 Optional 'man' extensions
      Custom headers and footers
      Ultrix-specific man macros
      Simple example
  4.2 'mdoc'
  4.3 'me'
  4.4 'mm'
  4.5 'mom'
  4.6 'ms'
    4.6.1 Introduction
      4.6.1.1 Basic information
    4.6.2 Document Structure
    4.6.3 Document Control Settings
      Margin settings
      Titles (headers, footers)
      Text settings
      Paragraph settings
      Heading settings
      Footnote settings
      Display settings
      Other settings
    4.6.4 Document Description Macros
    4.6.5 Body Text
      4.6.5.1 Text settings
      4.6.5.2 Typographical symbols
      4.6.5.3 Paragraphs
      4.6.5.4 Headings
      4.6.5.5 Typeface and decoration
      4.6.5.6 Lists
      4.6.5.7 Indented regions
      4.6.5.8 Keeps, boxed keeps, and displays
      4.6.5.9 Tables, figures, equations, and references
      4.6.5.10 Footnotes
      4.6.5.11 Language and localization
    4.6.6 Page layout
      4.6.6.1 Headers and footers
      4.6.6.2 Tab stops
      4.6.6.3 Margins
      4.6.6.4 Multiple columns
      4.6.6.5 Creating a table of contents
    4.6.7 Differences from AT&T 'ms'
      4.6.7.1 Unix Version 7 'ms' macros not implemented by 'groff' 'ms'
    4.6.8 Legacy Features
      AT&T accent mark strings
      Berkeley accent mark and glyph strings
    4.6.9 Naming Conventions
5 GNU 'troff' Reference
  5.1 Text
    5.1.1 Filling
    5.1.2 Sentences
    5.1.3 Hyphenation
    5.1.4 Breaking
    5.1.5 Adjustment
    5.1.6 Tabs and Leaders
    5.1.7 Requests and Macros
    5.1.8 Macro Packages
    5.1.9 Input Encodings
    5.1.10 Input Conventions
  5.2 Page Geometry
  5.3 Measurements
    5.3.1 Motion Quanta
    5.3.2 Default Units
  5.4 Numeric Expressions
  5.5 Identifiers
  5.6 Formatter Instructions
    5.6.1 Control Characters
    5.6.2 Invoking Requests
    5.6.3 Calling Macros
    5.6.4 Using Escape Sequences
    5.6.5 Delimiters
  5.7 Comments
  5.8 Registers
    5.8.1 Setting Registers
    5.8.2 Interpolating Registers
    5.8.3 Auto-increment
    5.8.4 Assigning Register Formats
    5.8.5 Built-in Registers
  5.9 Manipulating Filling and Adjustment
  5.10 Manipulating Hyphenation
  5.11 Manipulating Spacing
  5.12 Tabs and Fields
    5.12.1 Leaders
    5.12.2 Fields
  5.13 Character Translations
  5.14 'troff' and 'nroff' Modes
  5.15 Line Layout
  5.16 Line Continuation
  5.17 Page Layout
  5.18 Page Control
  5.19 Fonts and Symbols
    5.19.1 Changing Fonts
    5.19.2 Font Families
    5.19.3 Font Positions
    5.19.4 Using Symbols
    5.19.5 Character Classes
    5.19.6 Special Fonts
    5.19.7 Artificial Fonts
    5.19.8 Ligatures and Kerning
    5.19.9 Dummy Characters
  5.20 Manipulating Type Size and Vertical Spacing
    5.20.1 Changing the Type Size
    5.20.2 Changing the Vertical Spacing
    5.20.3 Using Fractional Type Sizes
  5.21 Colors
  5.22 Strings
  5.23 Conditionals and Loops
    5.23.1 Operators in Conditionals
    5.23.2 if-then
    5.23.3 if-else
    5.23.4 Conditional Blocks
    5.23.5 while
  5.24 Writing Macros
    5.24.1 Parameters
    5.24.2 Copy Mode
  5.25 Page Motions
  5.26 Drawing Requests
  5.27 Deferring Output
  5.28 Traps
    5.28.1 Vertical Position Traps
      5.28.1.1 Page Location Traps
      5.28.1.2 Diversion Traps
    5.28.2 Input Line Traps
    5.28.3 Blank Line Traps
    5.28.4 Leading Space Traps
    5.28.5 End-of-input Traps
  5.29 Diversions
  5.30 Punning Names
  5.31 Environments
  5.32 Suppressing Output
  5.33 I/O
  5.34 Postprocessor Access
  5.35 Miscellaneous
  5.36 'gtroff' Internals
  5.37 Debugging
    5.37.1 Warnings
  5.38 Implementation Differences
    5.38.1 Safer Mode
    5.38.2 Compatibility Mode
    5.38.3 Other Differences
6 File Formats
  6.1 'gtroff' Output
    6.1.1 Language Concepts
      6.1.1.1 Separation
      6.1.1.2 Argument Units
      6.1.1.3 Document Parts
    6.1.2 Command Reference
      6.1.2.1 Comment Command
      6.1.2.2 Simple Commands
      6.1.2.3 Graphics Commands
      6.1.2.4 Device Control Commands
      6.1.2.5 Obsolete Command
    6.1.3 Intermediate Output Examples
    6.1.4 Output Language Compatibility
  6.2 Device and Font Description Files
    6.2.1 'DESC' File Format
    6.2.2 Font Description File Format
Appendix A Copying This Manual
Appendix B Request Index
Appendix C Escape Sequence Index
Appendix D Operator Index
Appendix E Register Index
Appendix F Macro Index
Appendix G String Index
Appendix H File Keyword Index
Appendix I Program and File Index
Appendix J Concept Index
GNU 'troff'
***********

This manual documents GNU 'troff' version 1.23.0.

   Copyright  1994-2023 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with no Invariant Sections, no Front-Cover Texts, and
     no Back-Cover Texts.  A copy of the license is included in the
     section entitled "GNU Free Documentation License".

1 Introduction
**************

GNU 'roff' (or 'groff') is a programming system for typesetting
documents.  It is highly flexible and has been used extensively for over
thirty years.

1.1 Background
==============

M. Douglas McIlroy, formerly of AT&T Bell Laboratories and present at
the creation of the Unix operating system, offers an authoritative
historical summary.

     The prime reason for Unix was the desire of Ken [Thompson], Dennis
     [Ritchie], and Joe Ossanna to have a pleasant environment for
     software development.  The fig leaf that got the nod from ...
     management was that an early use would be to develop a
     "stand-alone" word-processing system for use in typing pools and
     secretarial offices.  Perhaps they had in mind "dedicated", as
     distinct from "stand-alone"; that's what eventuated in various
     cases, most notably in the legal/patent department and in the AT&T
     CEO's office.

     Both those systems were targets of opportunity, not foreseen from
     the start.  When Unix was up and running on the PDP-11, Joe got
     wind of the legal department having installed a commercial word
     processor.  He went to pitch Unix as an alternative and clinched a
     trial by promising to make 'roff' able to number lines by tomorrow
     in order to fulfill a patent-office requirement that the commercial
     system did not support.

     Modems were installed so legal-department secretaries could try the
     Research machine.  They liked it and Joe's superb customer service.
     Soon the legal department got a system of their own.  Joe went on
     to create 'nroff' and 'troff'.  Document preparation became a
     widespread use of Unix, but no stand-alone word-processing system
     was ever undertaken.

   A history relating 'groff' to its predecessors 'roff', 'nroff', and
'troff' is available in the 'roff(7)' man page.

1.2 What Is 'groff'?
====================

'groff' (GNU 'roff') is a typesetting system that reads plain text input
files that include formatting commands to produce output in PostScript,
PDF, HTML, DVI, or other formats, or for display to a terminal.
Formatting commands can be low-level typesetting primitives, macros from
a supplied package, or user-defined macros.  All three approaches can be
combined.

   A reimplementation and extension of the typesetter from AT&T Unix,
'groff' is present on most POSIX systems owing to its long association
with Unix manuals (including man pages).  It and its predecessor are
notable for their production of several best-selling software
engineering texts.  'groff' is capable of producing typographically
sophisticated documents while consuming minimal system resources.

1.3 'groff' Capabilities
========================

So what exactly is 'groff' capable of doing?  'groff' provides a wide
range of low-level text formatting operations.  Using these, it is
possible to perform a wide range of formatting tasks, such as footnotes,
table of contents, multiple columns, etc.  Here's a list of the most
important operations supported by 'groff':

   * text filling, adjustment, and centering

   * hyphenation

   * page control

   * font and glyph size control

   * vertical spacing (e.g., double-spacing)

   * line length and indenting

   * macros, strings, diversions, and traps

   * registers

   * tabs, leaders, and fields

   * input and output conventions and character translation

   * overstrike, bracket, line drawing, and zero-width functions

   * local horizontal and vertical motions and the width function

   * three-part titles

   * output line numbering

   * conditional acceptance of input

   * environment switching

   * insertions from the standard input

   * input/output file switching

   * output and error messages

1.4 Macro Packages
==================

Since 'groff' provides such low-level facilities, it can be quite
difficult to use by itself.  However, 'groff' provides a "macro"
facility to specify how certain routine operations, such as starting
paragraphs, or printing headers and footers, should be done.  These
macros can be collected together into a "macro package".  There are a
number of macro packages available; the most common (and the ones
described in this manual) are 'man', 'mdoc', 'me', 'ms', and 'mm'.

1.5 Preprocessors
=================

Although 'groff' provides most functions needed to format a document,
some operations would be unwieldy (e.g., to draw pictures).  Therefore,
programs called "preprocessors" were written that understand their own
language and produce the necessary 'groff' operations.  These
preprocessors are able to differentiate their own input from the rest of
the document via markers.

   To use a preprocessor, Unix pipes are used to feed the output from
the preprocessor into 'groff'.  Any number of preprocessors may be used
on a given document; in this case, the preprocessors are linked together
into one pipeline.  However, with 'groff', the user does not need to
construct the pipe, but only tell 'groff' what preprocessors to use.

   'groff' currently has preprocessors for producing tables ('tbl'),
typesetting equations ('eqn'), drawing pictures ('pic' and 'grn'),
processing bibliographies ('refer'), and drawing chemical structures
('chem').  An associated program that is useful when dealing with
preprocessors is 'soelim'.

   A free implementation of 'grap', a preprocessor for drawing graphs,
can be obtained as an extra package; 'groff' can use 'grap' also.

   Unique to 'groff' is the 'preconv' preprocessor that enables 'groff'
to handle documents in various input encodings.

   Other preprocessors exist, but, unfortunately, no free
implementations are available.  Among them is a preprocessor for drawing
mathematical pictures ('ideal').

1.6 Output Devices
==================

'groff' produces device-independent code that may be fed into a
postprocessor to produce output for a particular device.  Currently,
'groff' has postprocessors for PostScript devices, character terminals,
X11 (for previewing), DVI, HP LaserJet 4 and Canon LBP printers (which
use CaPSL), HTML, XHTML, and PDF.

1.7 Installation
================

Installation procedures are documented by the files 'INSTALL',
'INSTALL.extra', and 'INSTALL.REPO' in the 'groff' source distribution.

1.8 Conventions Used in This Manual
===================================

We apply the term "groff" to the language documented here, the GNU
implementation of the overall system, the project that develops that
system, and the command of that name.  In the first sense, 'groff' is an
extended dialect of the 'roff' language, for which many similar
implementations exist.

   The 'roff' language features several major categories for which many
items are predefined.  Presentations of these items feature the form in
which the item is most commonly used on the left, and, aligned to the
right margin, the name of the category in brackets.

 -- Register: \n[example]
     The register 'example' is one that that 'groff' _doesn't_
     predefine.  You can create it yourself, though; see *note Setting
     Registers::.

   To make this document useful as a reference and not merely amiable
bedtime reading, we tend to present these syntax items in exhaustive
detail when they arise.  References to topics discussed later in the
text are frequent; skip material you don't understand yet.

   We use Texinfo's "result" (=>) and error-> notations to present
output written to the standard output and standard error streams,
respectively.  Diagnostic messages from the GNU 'troff' formatter and
other programs are examples of the latter, but the formatter can also be
directed to write user-specified messages to the standard error stream.
The notation then serves to identify the output stream and does not
necessarily mean that an error has occurred.(1)  (*note Conventions Used
in This Manual-Footnote-1::)

     $ echo "Twelve o'clock and" | groff -Tascii | sed '/^$/d'
         => Twelve o'clock and
     $ echo '.tm all is well.' | groff > /dev/null
         error-> all is well.

   Sometimes we use => somewhat abstractly to represent formatted text
that you will need to use a PostScript or PDF viewer program (or a
printer) to observe.  While arguably an abuse of notation, we think this
preferable to requiring the reader to understand the syntax of these
page description languages.

   We also present diagnostic messages in an abbreviated form, often
omitting the name of the program issuing them, the input file name, and
line number or other positional information when such data do not serve
to illuminate the topic under discussion.

   Most examples are of 'roff' language input that would be placed in a
text file.  Occasionally, we start an example with a '$' character to
indicate a shell prompt, as seen above.

   You are encouraged to try the examples yourself, and to alter them to
better learn 'groff''s behavior.  Our examples frequently need to direct
the formatter to set a line length (with '.ll') that will fit within the
page margins of this manual.  We mention this so that you know why it is
there before we discuss the 'll' request formally.(2)  (*note
Conventions Used in This Manual-Footnote-2::)

   (1) Unix and related operating systems distinguish standard output
and standard error streams _because_ of 'troff':
<https://minnie.tuhs.org/pipermail/tuhs/2013-December/006113.html>.

   (2) *Note Line Layout::.

1.9 Credits
===========

Large portions of this manual were taken from existing documents--most
notably, the manual pages for the 'groff' package by James Clark, and
Eric Allman's papers on the 'me' macro package.  Larry Kollar
contributed much of the material on the 'ms' macro package.

2 Invoking 'groff'
******************

This chapter focuses on how to invoke the 'groff' front end.  This front
end takes care of the details of constructing the pipeline among the
preprocessors, 'gtroff' and the postprocessor.

   It has become a tradition that GNU programs get the prefix 'g' to
distinguish them from their original counterparts provided by the host
(*note Environment::).  Thus, for example, 'geqn' is GNU 'eqn'.  On
operating systems like GNU/Linux or the Hurd, which don't contain
proprietary versions of 'troff', and on MS-DOS/MS-Windows, where 'troff'
and associated programs are not available at all, this prefix is omitted
since GNU 'troff' is the only incarnation of 'troff' used.  Exception:
'groff' is never replaced by 'roff'.

   In this document, we consequently say 'gtroff' when talking about the
GNU 'troff' program.  All other implementations of 'troff' are called
AT&T 'troff', which is the common origin of almost all 'troff'
implementations(1) (*note Invoking groff-Footnote-1::) (with more or
less compatible changes).  Similarly, we say 'gpic', 'geqn', and so on.

   (1) Besides 'groff', 'neatroff' is an exception.

2.1 Options
===========

'groff' normally runs the 'gtroff' program and a postprocessor
appropriate for the selected device.  The default device is 'ps' (but it
can be changed when 'groff' is configured and built).  It can optionally
preprocess with any of 'gpic', 'geqn', 'gtbl', 'ggrn', 'grap', 'gchem',
'grefer', 'gsoelim', or 'preconv'.

   This section only documents options to the 'groff' front end.  Many
of the arguments to 'groff' are passed on to 'gtroff', therefore those
are also included.  Arguments to preprocessors and output drivers can be
found in the man pages 'gpic(1)', 'geqn(1)', 'gtbl(1)', 'ggrn(1)',
'grefer(1)', 'gchem(1)', 'gsoelim(1)', 'preconv(1)', 'grotty(1)',
'grops(1)', 'gropdf(1)', 'grohtml(1)', 'grodvi(1)', 'grolj4(1)',
'grolbp(1)', and 'gxditview(1)'.

   The command-line format for 'groff' is:

     groff [ -abceghijklpstvzCEGNRSUVXZ ] [ -dCS ] [ -DARG ]
           [ -fFAM ] [ -FDIR ] [ -IDIR ] [ -KARG ]
           [ -LARG ] [ -mNAME ] [ -MDIR ] [ -nNUM ]
           [ -oLIST ] [ -PARG ] [ -rCN ] [ -TDEV ]
           [ -wNAME ] [ -WNAME ] [ FILES... ]

   The command-line format for 'gtroff' is as follows.

     gtroff [ -abcivzCERU ] [ -dCS ] [ -fFAM ] [ -FDIR ]
            [ -mNAME ] [ -MDIR ] [ -nNUM ] [ -oLIST ]
            [ -rCN ] [ -TNAME ] [ -wNAME ] [ -WNAME ]
            [ FILES... ]

Obviously, many of the options to 'groff' are actually passed on to
'gtroff'.

   Options without an argument can be grouped behind a single '-'.  A
filename of '-' denotes the standard input.  Whitespace is permitted
between an option and its argument.

   The 'grog' command can be used to guess the correct 'groff' command
to format a file.  See its man page 'grog(1)'; type 'man grog' at the
command line to view it.

   'groff''s command-line options are as follows.

'-a'
     Generate a plain text approximation of the typeset output.  The
     read-only register '.A' is set to 1.  *Note Built-in Registers::.
     This option produces a sort of abstract preview of the formatted
     output.

        * Page breaks are marked by a phrase in angle brackets; for
          example, '<beginning of page>'.

        * Lines are broken where they would be in the formatted output.

        * A horizontal motion of any size is represented as one space.
          Adjacent horizontal motions are not combined.  Inter-sentence
          space nodes (those arising from the second argument to the
          'ss' request) are not represented.

        * Vertical motions are not represented.

        * Special characters are rendered in angle brackets; for
          example, the default soft hyphen character appears as '<hy>'.

     The above description should not be considered a specification; the
     details of '-a' output are subject to change.

'-b'
     Write a backtrace reporting the state of 'gtroff''s input parser to
     the standard error stream with each diagnostic message.  The line
     numbers given in the backtrace might not always be correct, because
     'gtroff''s idea of line numbers can be confused by requests that
     append to macros.

'-c'
     Start with color output disabled.

'-C'
     Enable AT&T 'troff' compatibility mode; implies '-c'.  *Note
     Implementation Differences::, for the list of incompatibilities
     between 'groff' and AT&T 'troff'.

'-dCTEXT'
'-dSTRING=TEXT'
     Define 'roff' string C or STRING as T or TEXT.  C must be one
     character; STRING can be of arbitrary length.  Such string
     assignments happen before any macro file is loaded, including the
     startup file.  Due to 'getopt_long' limitations, C cannot be, and
     STRING cannot contain, an equals sign, even though that is a valid
     character in a 'roff' identifier.

'-DENC'
     Set fallback input encoding used by 'preconv' to ENC; implies '-k'.

'-e'
     Run 'geqn' preprocessor.

'-E'
     Inhibit 'gtroff' error messages.  This option does _not_ suppress
     messages sent to the standard error stream by documents or macro
     packages using 'tm' or related requests.

'-fFAM'
     Use FAM as the default font family.  *Note Font Families::.

'-FDIR'
     Search in directory 'DIR' for the selected output device's
     directory of device and font description files.  See the
     description of 'GROFF_FONT_PATH' in *note Environment:: below for
     the default search locations and ordering.

'-g'
     Run 'ggrn' preprocessor.

'-G'
     Run 'grap' preprocessor; implies '-p'.

'-h'
     Display a usage message and exit.

'-i'
     Read the standard input after all the named input files have been
     processed.

'-IDIR'
     Search the directory DIR for files named in several contexts;
     implies '-g' and '-s'.

        * 'gsoelim' replaces 'so' requests with the contents of their
          file name arguments.

        * 'gtroff' searches for files named as operands in its command
          line and as arguments to 'psbb', 'so', and 'soquiet' requests.

        * Output drivers may search for files; for instance, 'grops'
          looks for files named in '\X'ps: import ...'', '\X'ps: file
          ...'', and '\X'pdf: pdfpic ...'' device control escape
          sequences.

     This option may be specified more than once; the directories are
     searched in the order specified.  If you want to search the current
     directory before others, add '-I .' at the desired place.  The
     current working directory is otherwise searched last.  '-I' works
     similarly to, and is named for, the "include" option of Unix C
     compilers.

     '-I' options are passed to 'gsoelim', 'gtroff', and output drivers;
     with the flag letter changed to '-M', they are also passed to
     'ggrn'.

'-j'
     Run 'gchem' preprocessor.  Implies '-p'.

'-k'
     Run 'preconv' preprocessor.  Refer to its man page for its behavior
     if neither of 'groff''s '-K' or '-D' options is also specified.

'-KENC'
     Set input encoding used by 'preconv' to ENC; implies '-k'.

'-l'
     Send the output to a spooler for printing.  The 'print' directive
     in the device description file specifies the default command to be
     used; see *note Device and Font Description Files::.  See options
     '-L' and '-X'.

'-LARG'
     Pass ARG to the print spooler program.  If multiple ARGs are
     required, pass each with a separate '-L' option.  'groff' does not
     prefix an option dash to ARG before passing it to the spooler
     program.

'-mNAME'
     Process the file 'NAME.tmac' prior to any input files.  If not
     found, 'tmac.NAME' is attempted.  NAME (in both arrangements) is
     presumed to be a macro file; see the description of
     'GROFF_TMAC_PATH' in *note Environment:: below for the default
     search locations and ordering.  This option and its argument are
     also passed to 'geqn', 'grap', and 'ggrn'.

'-MDIR'
     Search directory 'DIR' for macro files; see the description of
     'GROFF_TMAC_PATH' in *note Environment:: below for the default
     search locations and ordering.  This option and its argument are
     also passed to 'geqn', 'grap', and 'ggrn'.

'-nNUM'
     Number the first page NUM.

'-N'
     Prohibit newlines between 'eqn' delimiters: pass '-N' to 'geqn'.

'-oLIST'
     Output only pages in LIST, which is a comma-separated list of page
     ranges; 'N' means page N, 'M-N' means every page between M and N,
     '-N' means every page up to N, 'N-' means every page from N on.
     'gtroff' stops processing and exits after formatting the last page
     enumerated in LIST.

'-p'
     Run 'gpic' preprocessor.

'-PARG'
     Pass ARG to the postprocessor.  If multiple ARGs are required, pass
     each with a separate '-P' option.  'groff' does not prefix an
     option dash to ARG before passing it to the postprocessor.

'-rCNUMERIC-EXPRESSION'
'-rREGISTER=EXPR'
     Set 'roff' register C or REGISTER to the value NUMERIC-EXPRESSION
     (*note Numeric Expressions::).  C must be one character; REGISTER
     can be of arbitrary length.  Such register assignments happen
     before any macro file is loaded, including the startup file.  Due
     to 'getopt_long' limitations, C cannot be, and REGISTER cannot
     contain, an equals sign, even though that is a valid character in a
     'roff' identifier.

'-R'
     Run 'grefer' preprocessor.  No mechanism is provided for passing
     arguments to 'grefer' because most 'grefer' options have equivalent
     language elements that can be specified within the document.

     'gtroff' also accepts a '-R' option, which is not accessible via
     'groff'.  This option prevents the loading of the 'troffrc' and
     'troffrc-end' files.

'-s'
     Run 'gsoelim' preprocessor.

'-S'
     Operate in "safer" mode; see '-U' below for its opposite.  For
     security reasons, safer mode is enabled by default.

'-t'
     Run 'gtbl' preprocessor.

'-TDEV'
     Direct 'gtroff' to format the input for the output device DEV.
     'groff' then calls an output driver to convert 'gtroff''s output to
     a form appropriate for DEV.  The following output devices are
     available.

     'ps'
          For PostScript printers and previewers.

     'pdf'
          For PDF viewers or printers.

     'dvi'
          For TeX DVI format.

     'X75'
          For a 75dpi X11 previewer.

     'X75-12'
          For a 75dpi X11 previewer with a 12-point base font in the
          document.

     'X100'
          For a 100dpi X11 previewer.

     'X100-12'
          For a 100dpi X11 previewer with a 12-point base font in the
          document.

     'ascii'
          For typewriter-like devices using the (7-bit) ASCII (ISO 646)
          character set.

     'latin1'
          For typewriter-like devices that support the Latin-1
          (ISO 8859-1) character set.

     'utf8'
          For typewriter-like devices that use the Unicode (ISO 10646)
          character set with UTF-8 encoding.

     'cp1047'
          For typewriter-like devices that use the EBCDIC encoding IBM
          code page 1047.

     'lj4'
          For HP LaserJet4-compatible (or other PCL5-compatible)
          printers.

     'lbp'
          For Canon CaPSL printers (LBP-4 and LBP-8 series laser
          printers).

     'html'
     'xhtml'
          To produce HTML and XHTML output, respectively.  This driver
          consists of two parts, a preprocessor ('pre-grohtml') and a
          postprocessor ('post-grohtml').

     The predefined GNU 'troff' string '.T' contains the name of the
     output device; the read-only register '.T' is set to 1 if this
     option is used (which is always true if 'groff' is used to call GNU
     'troff').  *Note Built-in Registers::.

     The postprocessor to be used for a device is specified by the
     'postpro' command in the device description file.  (*Note Device
     and Font Description Files::.)  This can be overridden with the
     '-X' option.

'-U'
     Operate in "unsafe mode", which enables the 'open', 'opena', 'pi',
     'pso', and 'sy' requests.  These requests are disabled by default
     because they allow an untrusted input document to write to
     arbitrary file names and run arbitrary commands.  This option also
     adds the current directory to the macro package search path; see
     the '-m' option above.  '-U' is passed to 'gpic' and 'gtroff'.

'-v'
     Write version information for 'groff' and all programs run by it to
     the standard output stream; that is, the given command line is
     processed in the usual way, passing '-v' to the formatter and any
     pre- or postprocessors invoked.

'-V'
     Output the pipeline that would be run by 'groff' (as a wrapper
     program) to the standard output stream, but do not execute it.  If
     given more than once, the pipeline is both written to the standard
     error stream and run.

'-wCATEGORY'
     Enable warnings in CATEGORY.  Categories are listed in *note
     Warnings::.

'-WCATEGORY'
     Inhibit warnings in CATEGORY.  Categories are listed in *note
     Warnings::.

'-X'
     Use 'gxditview' instead of the usual postprocessor to (pre)view a
     document on an X11 display.  Combining this option with '-Tps' uses
     the font metrics of the PostScript device, whereas the '-TX75' and
     '-TX100' options use the metrics of X11 fonts.

'-z'
     Suppress formatted output from 'gtroff'.

'-Z'
     Disable postprocessing.  'gtroff' output will appear on the
     standard output stream (unless suppressed with '-z'; see *note
     gtroff Output:: for a description of this format.

2.2 Environment
===============

There are also several environment variables (of the operating system,
not within 'gtroff') that can modify the behavior of 'groff'.

'GROFF_BIN_PATH'
     This search path, followed by 'PATH', is used for commands executed
     by 'groff'.

'GROFF_COMMAND_PREFIX'
     If this is set to X, then 'groff' runs 'Xtroff' instead of
     'gtroff'.  This also applies to 'tbl', 'pic', 'eqn', 'grn', 'chem',
     'refer', and 'soelim'.  It does not apply to 'grops', 'grodvi',
     'grotty', 'pre-grohtml', 'post-grohtml', 'preconv', 'grolj4',
     'gropdf', and 'gxditview'.

     The default command prefix is determined during the installation
     process.  If a non-GNU 'troff' system is found, prefix 'g' is used,
     none otherwise.

'GROFF_ENCODING'
     The value of this variable is passed to the 'preconv'
     preprocessor's '-e' option to select the character encoding of
     input files.  This variable's existence implies the 'groff' option
     '-k'.  If set but empty, 'groff' calls 'preconv' without an '-e'
     option.  'groff''s '-K' option overrides 'GROFF_ENCODING'.  See the
     'preconv(7)' man page; type 'man preconv' at the command line to
     view it.

'GROFF_FONT_PATH'
     A list of directories in which to seek the selected output device's
     directory of device and font description files.  GNU 'troff' will
     search directories given as arguments to any specified '-F' options
     before these, and a built-in list of directories after them.  *Note
     Font Directories:: and the 'troff(1)' or 'gtroff(1)' man pages.

'GROFF_TMAC_PATH'
     A list of directories in which to seek macro files.  GNU 'troff'
     will search directories given as arguments to any specified '-M'
     options before these, and a built-in list of directories after
     them.  *Note Macro Directories:: and the 'troff(1)' or 'gtroff(1)'
     man pages.

'GROFF_TMPDIR'
     The directory in which 'groff' creates temporary files.  If this is
     not set and 'TMPDIR' is set, temporary files are created in that
     directory.  Otherwise temporary files are created in a
     system-dependent default directory (on Unix and GNU/Linux systems,
     this is usually '/tmp').  'grops', 'grefer', 'pre-grohtml', and
     'post-grohtml' can create temporary files in this directory.

'GROFF_TYPESETTER'
     Sets the default output device.  If empty or not set, a build-time
     default (often 'ps') is used.  The '-TDEV' option overrides
     'GROFF_TYPESETTER'.

'SOURCE_DATE_EPOCH'
     A timestamp (expressed as seconds since the Unix epoch) to use as
     the output creation timestamp in place of the current time.  The
     time is converted to human-readable form using 'localtime(3)' when
     the formatter starts up and stored in registers usable by documents
     and macro packages (*note Built-in Registers::).

'TZ'
     The time zone to use when converting the current time (or value of
     'SOURCE_DATE_EPOCH') to human-readable form; see 'tzset(3)'.

   MS-DOS and MS-Windows ports of 'groff' use semicolons, rather than
colons, to separate the directories in the lists described above.

2.3 Macro Directories
=====================

All macro file names must be named 'NAME.tmac' or 'tmac.NAME' to make
the '-mNAME' command-line option work.  The 'mso' request doesn't have
this restriction; any file name can be used, and 'gtroff' won't try to
append or prepend the 'tmac' string.

   Macro files are kept in the "tmac directories", all of which
constitute the "tmac path".  The elements of the search path for macro
files are (in that order):

   * The directories specified with 'gtroff''s or 'groff''s '-M'
     command-line option.

   * The directories given in the 'GROFF_TMAC_PATH' environment
     variable.

   * The current directory (only if in unsafe mode using the '-U'
     command-line switch).

   * The home directory.

   * A platform-dependent directory, a site-specific
     (platform-independent) directory, and the main tmac directory; the
     default locations are

          /usr/local/lib/groff/site-tmac
          /usr/local/share/groff/site-tmac
          /usr/local/share/groff/1.23.0/tmac

     assuming that the version of 'groff' is 1.23.0, and the
     installation prefix was '/usr/local'.  It is possible to fine-tune
     those directories during the installation process.

2.4 Font Directories
====================

Basically, there is no restriction how font files for 'groff' are named
and how long font names are; however, to make the font family mechanism
work (*note Font Families::), fonts within a family should start with
the family name, followed by the shape.  For example, the Times family
uses 'T' for the family name and 'R', 'B', 'I', and 'BI' to indicate the
shapes 'roman', 'bold', 'italic', and 'bold italic', respectively.  Thus
the final font names are 'TR', 'TB', 'TI', and 'TBI'.

   All font files are kept in the "font directories", which constitute
the "font path".  The file search functions always append the directory
'dev'NAME, where NAME is the name of the output device.  Assuming, say,
DVI output, and '/foo/bar' as a font directory, the font files for
'grodvi' must be in '/foo/bar/devdvi'.

   The elements of the search path for font files are (in that order):

   * The directories specified with 'gtroff''s or 'groff''s '-F'
     command-line option.  All device drivers and some preprocessors
     also have this option.

   * The directories given in the 'GROFF_FONT_PATH' environment
     variable.

   * A site-specific directory and the main font directory; the default
     locations are

          /usr/local/share/groff/site-font
          /usr/local/share/groff/1.23.0/font

     assuming that the version of 'groff' is 1.23.0, and the
     installation prefix was '/usr/local'.  It is possible to fine-tune
     those directories during the installation process.

2.5 Paper Format
================

In 'groff', the page dimensions for the formatter GNU 'troff' and for
output devices are handled separately.  *Note Page Layout::, for
vertical manipulation of the page size, and *Note Line Layout::, for
horizontal changes.  The 'papersize' macro package, normally loaded by
'troffrc' at startup, provides an interface for configuring page
dimensions by convenient names, like 'letter' or 'a4'; see
'groff_tmac(5)'.  The default used by the formatter depends on its build
configuration, but is usually one of the foregoing, as geographically
appropriate.

   It is up to each macro package to respect the page dimensions
configured in this way.

   For each output device, the size of the output medium can be set in
its 'DESC' file.  Most output drivers also recognize a command-line
option '-p' to override the default dimensions and an option '-l' to use
landscape orientation.  *Note DESC File Format::, for a description of
the 'papersize' keyword, which takes an argument of the same form as
'-p'.  The output driver's man page, such as 'grops(1)', may also be
helpful.

   'groff' uses the command-line option '-P' to pass options to
postprocessors; for example, use the following for PostScript output on
A4 paper in landscape orientation.

     groff -Tps -dpaper=a4l -P-pa4 -P-l -ms foo.ms > foo.ps

2.6 Invocation Examples
=======================

'roff' systems are best known for formatting man pages.  Once a 'man'
librarian program has located a man page, it may execute a 'groff'
command much like the following.

     groff -t -man -Tutf8 /usr/share/man/man1/groff.1

   The librarian will also pipe the output through a pager, which might
not interpret the SGR terminal escape sequences 'groff' emits for
boldface, underlining, or italics; see the 'grotty(1)' man page for a
discussion.

   To process a 'roff' input file using the preprocessors 'gtbl' and
'gpic' and the 'me' macro package in the way to which AT&T 'troff' users
were accustomed, one would type (or script) a pipeline.

     gpic foo.me | gtbl | gtroff -me -Tutf8 | grotty

   Using 'groff', this pipe can be shortened to an equivalent command.

     groff -p -t -me -T utf8 foo.me

   An even easier way to do this is to use 'grog' to guess the
preprocessor and macro options and execute the result by using the
command substitution feature of the shell.

     $(grog -Tutf8 foo.me)

   Each command-line option to a postprocessor must be specified with
any required leading dashes '-' because 'groff' passes the arguments
as-is to the postprocessor; this permits arbitrary arguments to be
transmitted.  For example, to pass a title to the 'gxditview'
postprocessor, the shell commands

     groff -X -P -title -P 'trial run' mydoc.t

and

     groff -X -Z mydoc.t | gxditview -title 'trial run' -

are equivalent.

3 Tutorial for Macro Users
**************************

Most users of the 'roff' language employ a macro package to format their
documents.  Successful macro packages tend to ease the composition
process; their users need not have mastered the full formatting
language, nor even some of its major features like diversions, traps,
and environments.  A familiarity with some basic concepts and mechanisms
common to macro packages (like "displays") remains helpful; this chapter
aims to bring you to this level.  If you prefer a meticulous and
comprehensive presentation, try *note GNU troff Reference:: instead.

3.1 Basics
==========

This section covers some of the basic concepts necessary to understand
how to use a macro package.(1)  (*note Basics-Footnote-1::) References
are made throughout to more detailed information, if desired.

   GNU 'troff' reads an input file prepared by the user and outputs a
formatted document suitable for publication or framing.  The input
consists of text, or words to be printed, and embedded commands
("requests" and "escape sequences"), which tell GNU 'troff' how to
format the output.  For more detail on this, see *note Formatter
Instructions::.

   The word "argument" is used in this chapter to mean a word or number
that appears on the same line as a request, and which modifies the
meaning of that request.  For example, the request

     .sp

spaces one line, but

     .sp 4

spaces four lines.  The number 4 is an argument to the 'sp' request,
which says to space four lines instead of one.  Arguments are separated
from the request and from each other by spaces (_no_ tabs).  *Note
Invoking Requests::.

   The primary function of 'gtroff' is to collect words from input
lines, fill output lines with those words, justify the right-hand margin
by inserting extra spaces in the line, and output the result.  For
example, the input:

     Now is the time
     for all good men
     to come to the aid
     of their party.
     Four score and seven
     years ago, etc.

is read, packed onto output lines, and justified to produce:

     Now is the time for all good men to come to the aid of their party.
     Four score and seven years ago, etc.

   Sometimes a new output line should be started even though the current
line is not yet full; for example, at the end of a paragraph.  To do
this it is possible to cause a "break", which starts a new output line.
Some requests cause a break automatically, as normally do blank input
lines and input lines beginning with a space.

   Not all input lines are text to be formatted.  Some input lines are
requests that describe how to format the text.  Requests always have a
period ('.') or an apostrophe (''') as the first character of the input
line.

   The text formatter also does more complex things, such as
automatically numbering pages, skipping over page boundaries, putting
footnotes in the correct place, and so forth.

   Here are a few hints for preparing text for input to 'gtroff'.

   * First, keep the input lines short.  Short input lines are easier to
     edit, and 'gtroff' packs words onto longer lines anyhow.

   * In keeping with this, it is helpful to begin a new line after every
     comma or phrase, since common corrections are to add or delete
     sentences or phrases.

   * End each sentence with two spaces--or better, start each sentence
     on a new line.  'gtroff' recognizes characters that usually end a
     sentence, and inserts inter-sentence space accordingly.

   * Do not hyphenate words at the end of lines--'gtroff' is smart
     enough to hyphenate words as needed, but is not smart enough to
     take hyphens out and join a word back together.  Also, words such
     as "mother-in-law" should not be broken over a line, since then a
     space can occur where not wanted, such as "mother- in-law".

   'gtroff' double-spaces output text automatically if you use the
request '.ls 2'.  Reactivate single-spaced mode by typing '.ls 1'.(2)
(*note Basics-Footnote-2::)

   A number of requests allow you to change the way the output is
arranged on the page, sometimes called the "layout" of the output page.

   The 'bp' request starts a new page, causing a line break.

   The request '.sp N' leaves N lines of blank space.  N can be omitted
(meaning skip a single line) or can be of the form Ni (for N inches) or
Nc (for N centimeters).  For example, the input:

     .sp 1.5i
     My thoughts on the subject
     .sp

leaves one and a half inches of space, followed by the line "My thoughts
on the subject", followed by a single blank line (more measurement units
are available, see *note Measurements::).

   Text lines can be centered by using the 'ce' request.  The line after
'ce' is centered (horizontally) on the page.  To center more than one
line, use '.ce N' (where N is the number of lines to center), followed
by the N lines.  To center many lines without counting them, type:

     .ce 1000
     lines to center
     .ce 0

The '.ce 0' request tells 'groff' to center zero more lines, in other
words, stop centering.

   All of these requests cause a break; that is, they always start a new
line.  To start a new line without performing any other action, use
'br'.

   (1) This section is derived from 'Writing Papers with nroff using
-me' by Eric P. Allman.

   (2) If you need finer granularity of the vertical space, use the
'pvs' request (*note Changing the Type Size::).

3.2 Common Features
===================

'gtroff' provides very low-level operations for formatting a document.
There are many common routine operations that are done in all documents.
These common operations are written into "macros" and collected into a
"macro package".

   All macro packages provide certain common capabilities that fall into
the following categories.

3.2.1 Paragraphs
----------------

One of the most common and most used capability is starting a paragraph.
There are a number of different types of paragraphs, any of which can be
initiated with macros supplied by the macro package.  Normally,
paragraphs start with a blank line and the first line indented, like the
text in this manual.  There are also block style paragraphs, which omit
the indentation:

     Some   men  look   at  constitutions   with  sanctimonious
     reverence, and deem them like the ark of the covenant, too
     sacred to be touched.

And there are also indented paragraphs, which begin with a tag or label
at the margin and the remaining text indented.

     one   This is  the first paragraph.  Notice  how the first
           line of  the resulting  paragraph lines up  with the
           other lines in the paragraph.

     longlabel
           This  paragraph   had  a  long   label.   The  first
           character of text on the first line does not line up
           with  the  text  on  second  and  subsequent  lines,
           although they line up with each other.

   A variation of this is a bulleted list.

     .     Bulleted lists start with a bullet.   It is possible
           to use other glyphs instead of the bullet.  In nroff
           mode using the ASCII character set for output, a dot
           is used instead of a real bullet.

3.2.2 Sections and Chapters
---------------------------

Most macro packages supply some form of section headers.  The simplest
kind is simply the heading on a line by itself in bold type.  Others
supply automatically numbered section heading or different heading
styles at different levels.  Some, more sophisticated, macro packages
supply macros for starting chapters and appendices.

3.2.3 Headers and Footers
-------------------------

Every macro package gives some way to manipulate the "headers" and
"footers" (also called "titles") on each page.  This is text put at the
top and bottom of each page, respectively, which contain data like the
current page number, the current chapter title, and so on.  Its
appearance is not affected by the running text.  Some packages allow for
different ones on the even and odd pages (for material printed in a book
form).

   The titles are called "three-part titles", that is, there is a
left-justified part, a centered part, and a right-justified part.  An
automatically generated page number may be put in any of these fields
with the '%' character (*note Page Layout::).

3.2.4 Page Layout
-----------------

Most macro packages let the user specify top and bottom margins and
other details about the appearance of the printed pages.

3.2.5 Displays
--------------

"Displays" are sections of text to be set off from the body of the
paper.  Major quotes, tables, and figures are types of displays, as are
all the examples used in this document.

   "Major quotes" are quotes that are several lines long, and hence are
set in from the rest of the text without quote marks around them.

   A "list" is an indented, single-spaced, unfilled display.  Lists
should be used when the material to be printed should not be filled and
justified like normal text, such as columns of figures or the examples
used in this paper.

   A "keep" is a display of lines that are kept on a single page if
possible.  An example for a keep might be a diagram.  Keeps differ from
lists in that lists may be broken over a page boundary whereas keeps are
not.

   "Floating keeps" move relative to the text.  Hence, they are good for
things that are referred to by name, such as "See figure 3".  A floating
keep appears at the bottom of the current page if it fits; otherwise, it
appears at the top of the next page.  Meanwhile, the surrounding text
'flows' around the keep, thus leaving no blank areas.

3.2.6 Footnotes and Annotations
-------------------------------

There are a number of requests to save text for later printing.

   "Footnotes" are printed at the bottom of the current page.

   "Delayed text" is very similar to a footnote except that it is
printed when called for explicitly.  This allows a list of references to
appear (for example) at the end of each chapter, as is the convention in
some disciplines.

   Most macro packages that supply this functionality also supply a
means of automatically numbering either type of annotation.

3.2.7 Table of Contents
-----------------------

"Tables of contents" are a type of delayed text having a tag (usually
the page number) attached to each entry after a row of dots.  The table
accumulates throughout the paper until printed, usually after the paper
has ended.  Many macro packages provide the ability to have several
tables of contents (e.g., a standard table of contents, a list of
tables, etc).

3.2.8 Indices
-------------

While some macro packages use the term "index", none actually provide
that functionality.  The facilities they call indices are actually more
appropriate for tables of contents.

   To produce a real index in a document, external tools like the
'makeindex' program are necessary.

3.2.9 Paper Formats
-------------------

Some macro packages provide stock formats for various kinds of
documents.  Many of them provide a common format for the title and
opening pages of a technical paper.  The 'mm' macros in particular
provide formats for letters and memoranda.

3.2.10 Multiple Columns
-----------------------

Some macro packages (but not 'man') provide the ability to have two or
more columns on a page.

3.2.11 Font and Size Changes
----------------------------

The built-in font and size functions are not always intuitive, so all
macro packages provide macros to make these operations simpler.

3.2.12 Predefined Strings
-------------------------

Most macro packages provide various predefined strings for a variety of
uses; examples are sub- and superscripts, printable dates, quotes and
various special characters.

3.2.13 Preprocessor Support
---------------------------

All macro packages provide support for various preprocessors and may
extend their functionality.

   For example, all macro packages mark tables (which are processed with
'gtbl') by placing them between 'TS' and 'TE' macros.  The 'ms' macro
package has an option, '.TS H', that prints a caption at the top of a
new page (when the table is too long to fit on a single page).

3.2.14 Configuration and Customization
--------------------------------------

Some macro packages provide means of customizing many of the details of
how the package behaves.  This ranges from setting the default type size
to changing the appearance of section headers.

4 Macro Packages
****************

This chapter surveys the "major" macro packages that come with 'groff'.
One, 'ms', is presented in detail.

   Major macro packages are also sometimes described as "full-service"
due to the breadth of features they provide and because more than one
cannot be used by the same document; for example

     groff -m man foo.man -m ms bar.doc

doesn't work.  Option arguments are processed before non-option
arguments; the above (failing) sample is thus reordered to

     groff -m man -m ms foo.man bar.doc

   Many auxiliary, or "minor" macro packages are also available.  They
may in general be used with any full-service macro package and handle a
variety of tasks from character encoding selection, to language
localization, to inlining of raster images.  See the 'groff_tmac(5)' man
page for a list.  Type 'man groff_tmac' at the command line to view it.

4.1 'man'
=========

The 'man' macro package is the most widely used and probably the most
important ever developed for 'troff'.  It is easy to use, and a vast
majority of manual pages ("man pages") are written in it.

   'groff''s implementation is documented in the 'groff_man(7)' man
page.  Type 'man groff_man' at the command line to view it.

4.1.1 Optional 'man' extensions
-------------------------------

Use the file 'man.local' for local extensions to the 'man' macros or for
style changes.

Custom headers and footers
..........................

In 'groff' versions 1.18.2 and later, you can specify custom headers and
footers by redefining the following macros in 'man.local'.

 -- Macro: .PT
     Control the content of the headers.  Normally, the header prints
     the command name and section number on either side, and the
     optional fifth argument to 'TH' in the center.

 -- Macro: .BT
     Control the content of the footers.  Normally, the footer prints
     the page number and the third and fourth arguments to 'TH'.

     Use the 'FT' register to specify the footer position.  The default
     is -0.5i.

Ultrix-specific man macros
..........................

The 'groff' source distribution includes a file named 'man.ultrix',
containing macros compatible with the Ultrix variant of 'man'.  Copy
this file into 'man.local' (or use the 'mso' request to load it) to
enable the following macros.

 -- Macro: .CT key
     Print '<CTRL/KEY>'.

 -- Macro: .CW
     Print subsequent text using a "constant-width" (monospaced)
     typeface (Courier roman).

 -- Macro: .Ds
     Begin a non-filled display.

 -- Macro: .De
     End a non-filled display started with 'Ds'.

 -- Macro: .EX [indent]
     Begin a non-filled display using a monospaced typeface (Courier
     roman).  Use the optional INDENT argument to indent the display.

 -- Macro: .EE
     End a non-filled display started with 'EX'.

 -- Macro: .G [text]
     Set TEXT in Helvetica.  If no text is present on the line where the
     macro is called, then the text of the next line appears in
     Helvetica.

 -- Macro: .GL [text]
     Set TEXT in Helvetica oblique.  If no text is present on the line
     where the macro is called, then the text of the next line appears
     in Helvetica Oblique.

 -- Macro: .HB [text]
     Set TEXT in Helvetica bold.  If no text is present on the line
     where the macro is called, then all text up to the next 'HB'
     appears in Helvetica bold.

 -- Macro: .TB [text]
     Identical to 'HB'.

 -- Macro: .MS title sect [punct]
     Set a man page reference in Ultrix format.  The TITLE is in Courier
     instead of italic.  Optional punctuation follows the section number
     without an intervening space.

 -- Macro: .NT [C] [title]
     Begin a note.  Print the optional title, or the word "Note",
     centered on the page.  Text following the macro makes up the body
     of the note, and is indented on both sides.  If the first argument
     is 'C', the body of the note is printed centered (the second
     argument replaces the word "Note" if specified).

 -- Macro: .NE
     End a note begun with 'NT'.

 -- Macro: .PN path [punct]
     Set the path name in a monospaced typeface (Courier roman),
     followed by optional punctuation.

 -- Macro: .Pn [punct] path [punct]
     If called with two arguments, identical to 'PN'.  If called with
     three arguments, set the second argument in a monospaced typeface
     (Courier roman), bracketed by the first and third arguments in the
     current font.

 -- Macro: .R
     Switch to roman font and turn off any underlining in effect.

 -- Macro: .RN
     Print the string '<RETURN>'.

 -- Macro: .VS [4]
     Start printing a change bar in the margin if the number '4' is
     specified.  Otherwise, this macro does nothing.

 -- Macro: .VE
     End printing the change bar begun by 'VS'.

Simple example
..............

The following example 'man.local' file alters the 'SH' macro to add some
extra vertical space before printing the heading.  Headings are printed
in Helvetica bold.

     .\" Make the heading fonts Helvetica
     .ds HF HB
     .
     .\" Put more space in front of headings.
     .rn SH SH-orig
     .de SH
     .  if t .sp (u;\\n[PD]*2)
     .  SH-orig \\$*
     ..

4.2 'mdoc'
==========

'groff''s implementation of the BSD 'doc' package for man pages is
documented in the 'groff_mdoc(7)' man page.  Type 'man groff_mdoc' at
the command line to view it.

4.3 'me'
========

'groff''s implementation of the BSD 'me' macro package is documented
using itself.  A tutorial, 'meintro.me', and reference, 'meref.me', are
available in 'groff''s documentation directory.  A 'groff_me(7)' man
page is also available and identifies the installation path for these
documents.  Type 'man groff_me' at the command line to view it.

   A French translation of the tutorial is available as 'meintro_fr.me'
and installed parallel to the English version.

4.4 'mm'
========

'groff''s implementation of the AT&T memorandum macro package is
documented in the 'groff_mm(7)' man page.  Type 'man groff_mm' at the
command line) to view it.

   A Swedish localization of 'mm' is also available; see
'groff_mmse(7)'.

4.5 'mom'
=========

The main documentation files for the 'mom' macros are in HTML format.
Additional, useful documentation is in PDF format.  See the 'groff(1)'
man page, section "Installation Directories", for their location.

   * 'toc.html' Entry point to the full mom manual.

   * 'macrolist.html' Hyperlinked index of macros with brief
     descriptions, arranged by category.

   * 'mom-pdf.pdf' PDF features and usage.

   The mom macros are in active development between 'groff' releases.
The most recent version, along with up-to-date documentation, is
available at <http://www.schaffter.ca/mom/mom-05.html>.

   The 'groff_mom(7)' man page (type 'man groff_mom' at the command
line) contains a partial list of available macros, however their usage
is best understood by consulting the HTML documentation.

4.6 'ms'
========

The 'ms' ("manuscript") package is suitable for the preparation of
letters, memoranda, reports, and books.  These 'groff' macros feature
cover page and table of contents generation, automatically numbered
headings, several paragraph styles, a variety of text styling options,
footnotes, and multi-column page layouts.  'ms' supports the 'tbl',
'eqn', 'pic', and 'refer' preprocessors for inclusion of tables,
mathematical equations, diagrams, and standardized bibliographic
citations.  This implementation is mostly compatible with the documented
interface and behavior of AT&T Unix Version 7 'ms'.  Many extensions
from 4.2BSD (Berkeley) and Tenth Edition Research Unix have been
recreated.

4.6.1 Introduction
------------------

The 'ms' macros are the oldest surviving package for 'roff' systems.(1)
(*note ms Introduction-Footnote-1::) While the 'man' package was
designed for brief reference documents, the 'ms' macros are also
suitable for longer works intended for printing and possible
publication.

   (1) While manual _pages_ are older, early ones used macros supplanted
by the 'man' package of Seventh Edition Unix (1979).  'ms' shipped with
Sixth Edition (1975) and was documented by Mike Lesk in a Bell Labs
internal memorandum.

4.6.1.1 Basic information
.........................

'ms' documents are plain text files; prepare them with your preferred
text editor.  If you're in a hurry to start, know that 'ms' needs one of
its macros called at the beginning of a document so that it can
initialize.  A "macro" is a formatting instruction to 'ms'.  Put a macro
call on a line by itself.  Use '.PP' if you want your paragraph's first
line to be indented, or '.LP' if you don't.

   After that, start typing normally.  It is a good practice to start
each sentence on a new line, or to put two spaces after sentence-ending
puncutation, so that the formatter knows where the sentence boundaries
are.  You can separate paragraphs with further paragraphing macros, or
with blank lines, and you can indent with tabs.  When you need one of
the features mentioned earlier (*note ms::), return to this part of the
manual.

   Format the document with the 'groff' command.  'nroff' can be useful
for previewing.

     $ editor radical.ms
     $ nroff -ww -z -ms radical.ms # check for errors
     $ nroff -ms radical.ms | less -R
     $ groff -T ps -ms radical.ms > radical.ps
     $ see radical.ps

   Our 'radical.ms' document might look like this.

     .LP
     Radical novelties are so disturbing that they tend to be
     suppressed or ignored, to the extent that even the
     possibility of their existence in general is more often
     denied than admitted.

     ->That's what Dijkstra said, anyway.

   'ms' exposes many aspects of document layout to user control via
'groff''s "registers" and "strings", which store numbers and text,
respectively.  Measurements in 'groff' are expressed with a suffix
called a "scaling unit".

'i'
     inches

'c'
     centimeters

'p'
     points (1/72 inch)

'P'
     picas (1/6 inch)

'v'
     vees; current vertical spacing

'm'
     ems; width of an "M" in the current font

'n'
     ens; one-half em

   Set registers with the 'nr' request and strings with the 'ds'
request.  "Requests" are like macro calls; they go on lines by
themselves and start with the "control character", a dot ('.').  The
difference is that they directly instruct the formatter program, rather
than the macro package.  We'll discuss a few as applicable.  It is wise
to specify a scaling unit when setting any register that represents a
length, size, or distance.

     .nr PS 10.5p \" Use 10.5-point type.
     .ds FAM P    \" Use Palatino font family.

In the foregoing, we see that '\"' begins a comment.  This is an example
of an "escape sequence", the other kind of formatting instruction.
Escape sequences can appear anywhere.  They begin with the escape
character ('\') and are followed by at least one more character.  'ms'
documents tend to use only a few of 'groff''s many requests and escape
sequences; see *note Request Index:: and *note Escape Sequence Index::
or the 'groff(7)' man page for complete lists.

'\"'
     Begin comment; ignore remainder of line.

'\n[REG]'
     Interpolate value of register REG.

'\*[STR]'
     Interpolate contents of string STR.

'\*S'
     abbreviation of '\*[S]'; the name S must be only one character

'\[CHAR]'
     Interpolate glyph of special character named CHAR.

'\&'
     dummy character

'\~'
     Insert an unbreakable space that is adjustable like a normal space.

'\|'
     Move horizontally by one-sixth em ("thin space").

   Prefix any words that start with a dot '.' or neutral apostrophe '''
with '\&' if they are at the beginning of an input line (or might become
that way in editing) to prevent them from being interpreted as macro
calls or requests.  Suffix '.', '?', and '!' with '\&' when needed to
cancel end-of-sentence detection.

     My exposure was \&.5 to \&.6 Sv of neutrons, said Dr.\&
     Wallace after the criticality incident.

4.6.2 Document Structure
------------------------

The 'ms' macro package expects a certain amount of structure: a
well-formed document contains at least one paragraphing or heading macro
call.  Longer documents have a structure as follows.

*Document type*
     Calling the 'RP' macro at the beginning of your document puts the
     document description (see below) on a cover page.  Otherwise, 'ms'
     places the information (if any) on the first page, followed
     immediately by the body text.  Some document types found in other
     'ms' implementations are specific to AT&T or Berkeley, and are not
     supported by 'groff' 'ms'.

*Format and layout*
     By setting registers and strings, you can configure your document's
     typeface, margins, spacing, headers and footers, and footnote
     arrangement.  *Note ms Document Control Settings::.

*Document description*
     A document description consists of any of: a title, one or more
     authors' names and affiliated institutions, an abstract, and a date
     or other identifier.  *Note ms Document Description Macros::.

*Body text*
     The main matter of your document follows its description (if any).
     'ms' supports highly structured text consisting of paragraphs
     interspersed with multi-level headings (chapters, sections,
     subsections, and so forth) and augmented by lists, footnotes,
     tables, diagrams, and similar material.  *Note ms Body Text::.

*Tables of contents*
     Macros enable the collection of entries for a table of contents (or
     index) as the material they discuss appears in the document.  You
     then call a macro to emit the table of contents at the end of your
     document.  The table of contents must necessarily follow the rest
     of the text since GNU 'troff' is a single-pass formatter; it thus
     cannot determine the page number of a division of the text until it
     has been set and output.  Since 'ms' was designed for the
     production of hard copy, the traditional procedure was to manually
     relocate the pages containing the table of contents between the
     cover page and the body text.  Today, page resequencing can be done
     in the digital domain with tools like 'pdfjam(1)'.  An index works
     similarly, but because it typically needs to be sorted after
     collection, its preparation requires separate processing.

4.6.3 Document Control Settings
-------------------------------

'ms' exposes many aspects of document layout to user control via 'groff'
requests.  To use them, you must understand how to define registers and
strings.

 -- Request: .nr reg value
     Set register REG to VALUE.  If REG doesn't exist, GNU 'troff'
     creates it.

 -- Request: .ds name contents
     Set string NAME to CONTENTS.

   A list of document control registers and strings follows.  For any
parameter whose default is unsatisfactory, define its register or string
before calling any 'ms' macro other than 'RP'.

Margin settings
...............

 -- Register: \n[PO]
     Defines the page offset (i.e., the left margin).

     Effective: next page.

     Default: Varies by output device and paper format; 1i is used for
     typesetters using U.S. letter paper, and zero for terminals.  *Note
     Paper Format::.

 -- Register: \n[LL]
     Defines the line length (i.e., the width of the body text).

     Effective: next paragraph.

     Default: Varies by output device and paper format; 6.5i is used for
     typesetters using U.S. letter paper (*note Paper Format::) and 65n
     on terminals.

 -- Register: \n[LT]
     Defines the title line length (i.e., the header and footer width).
     This is usually the same as 'LL', but need not be.

     Effective: next paragraph.

     Default: Varies by output device and paper format; 6.5i is used for
     typesetters using U.S. letter paper (*note Paper Format::) and 65n
     on terminals.

 -- Register: \n[HM]
     Defines the header margin height at the top of the page.

     Effective: next page.

     Default: 1i.

 -- Register: \n[FM]
     Defines the footer margin height at the bottom of the page.

     Effective: next page.

     Default: 1i.

Titles (headers, footers)
.........................

 -- String: \*[LH]
     Defines the text displayed in the left header position.

     Effective: next header.

     Default: empty.

 -- String: \*[CH]
     Defines the text displayed in the center header position.

     Effective: next header.

     Default: '-\n[%]-'.

 -- String: \*[RH]
     Defines the text displayed in the right header position.

     Effective: next header.

     Default: empty.

 -- String: \*[LF]
     Defines the text displayed in the left footer position.

     Effective: next footer.

     Default: empty.

 -- String: \*[CF]
     Defines the text displayed in the center footer position.

     Effective: next footer.

     Default: empty.

 -- String: \*[RF]
     Defines the text displayed in the right footer position.

     Effective: next footer.

     Default: empty.

Text settings
.............

 -- Register: \n[PS]
     Defines the type size of the body text.

     Effective: next paragraph.

     Default: 10p.

 -- Register: \n[VS]
     Defines the vertical spacing (type size plus leading).

     Effective: next paragraph.

     Default: 12p.

 -- Register: \n[HY]
     Defines the automatic hyphenation mode used with the 'hy' request.
     Setting 'HY' to 0 is equivalent to using the 'nh' request.  This is
     a Tenth Edition Research Unix extension.

     Effective: next paragraph.

     Default: 6.

 -- String: \*[FAM]
     Defines the font family used to typeset the document.  This is a
     GNU extension.

     Effective: next paragraph.

     Default: defined by the output device; often 'T' (*note ms Body
     Text::)

Paragraph settings
..................

 -- Register: \n[PI]
     Defines the indentation amount used by the 'PP', 'IP' (unless
     overridden by an optional argument), 'XP', and 'RS' macros.

     Effective: next paragraph.

     Default: 5n.

 -- Register: \n[PD]
     Defines the space between paragraphs.

     Effective: next paragraph.

     Default: 0.3v (1v on low-resolution devices).

 -- Register: \n[QI]
     Defines the indentation amount used on both sides of a paragraph
     set with the 'QP' or between the 'QS' and 'QE' macros.

     Effective: next paragraph.

     Default: 5n.

 -- Register: \n[PORPHANS]
     Defines the minimum number of initial lines of any paragraph that
     must be kept together to avoid isolated lines at the bottom of a
     page.  If a new paragraph is started close to the bottom of a page,
     and there is insufficient space to accommodate 'PORPHANS' lines
     before an automatic page break, then a page break is forced before
     the start of the paragraph.  This is a GNU extension.

     Effective: next paragraph.

     Default: 1.

Heading settings
................

 -- Register: \n[PSINCR]
     Defines an increment in type size to be applied to a heading at a
     lesser depth than that specified in 'GROWPS'.  The value of
     'PSINCR' should be specified in points with the p scaling unit and
     may include a fractional component; for example, '.nr PSINCR 1.5p'
     sets a type size increment of 1.5p.  This is a GNU extension.

     Effective: next heading.

     Default: 1p.

 -- Register: \n[GROWPS]
     Defines the heading depth above which the type size increment set
     by 'PSINCR' becomes effective.  For each heading depth less than
     the value of 'GROWPS', the type size is increased by 'PSINCR'.
     Setting 'GROWPS' to any value less than 2 disables the incremental
     heading size feature.  This is a GNU extension.

     Effective: next heading.

     Default: 0.

 -- Register: \n[HORPHANS]
     Defines the minimum number of lines of an immediately succeeding
     paragraph that should be kept together with any heading introduced
     by the 'NH' or 'SH' macros.  If a heading is placed close to the
     bottom of a page, and there is insufficient space to accommodate
     both the heading and at least 'HORPHANS' lines of the following
     paragraph, before an automatic page break, then the page break is
     forced before the heading.  This is a GNU extension.

     Effective: next paragraph.

     Default: 1.

 -- String: \*[SN-STYLE]
     Defines the style used to print numbered headings.  *Note Headings
     in ms::.  This is a GNU extension.

     Effective: next heading.

     Default: alias of 'SN-DOT'

Footnote settings
.................

 -- Register: \n[FI]
     Defines the footnote indentation.  This is a Berkeley extension.

     Effective: next footnote.

     Default: 2n.

 -- Register: \n[FF]
     Defines the format of automatically numbered footnotes, and those
     for which the 'FS' request is given a marker argument, at the
     bottom of a column or page.  This is a Berkeley extension.
     '0'
          Set an automatic number(1) (*note ms Document Control
          Settings-Footnote-1::) as a superscript (on typesetter
          devices) or surrounded by square brackets (on terminals).  The
          footnote paragraph is indented as with 'PP' if there is an
          'FS' argument or an automatic number, and as with 'LP'
          otherwise.  This is the default.

     '1'
          As '0', but set the marker as regular text and follow an
          automatic number with a period.

     '2'
          As '1', but without indentation (like 'LP').

     '3'
          As '1', but set the footnote paragraph with the marker hanging
          (like 'IP').

     Effective: next footnote.

     Default: 0.

 -- Register: \n[FPS]
     Defines the footnote type size.

     Effective: next footnote.

     Default: '\n[PS] - 2p'.

 -- Register: \n[FVS]
     Defines the footnote vertical spacing.

     Effective: next footnote.

     Default: '\n[FPS] + 2p'.

 -- Register: \n[FPD]
     Defines the footnote paragraph spacing.  This is a GNU extension.

     Effective: next footnote.

     Default: '\n[PD] / 2'.

 -- String: \*[FR]
     Defines the ratio of the footnote line length to the current line
     length.  This is a GNU extension.

     Effective: next footnote in single-column arrangements, next page
     otherwise.

     Default: '11/12'.

Display settings
................

 -- Register: \n[DD]
     Sets the display distance--the vertical spacing before and after a
     display, a 'tbl' table, an 'eqn' equation, or a 'pic' image.  This
     is a Berkeley extension.

     Effective: next display boundary.

     Default: 0.5v (1v on low-resolution devices).

 -- Register: \n[DI]
     Sets the default amount by which to indent a display started with
     'DS' and 'ID' without arguments, to '.DS I' without an indentation
     argument, and to equations set with '.EQ I'.  This is a GNU
     extension.

     Effective: next indented display.

     Default: 0.5i.

Other settings
..............

 -- Register: \n[MINGW]
     Defines the default minimum width between columns in a multi-column
     document.  This is a GNU extension.

     Effective: next page.

     Default: 2n.

 -- Register: \n[TC-MARGIN]
     Defines the width of the field in which page numbers are set in a
     table of contents entry; the right margin thus moves inboard by
     this amount.  This is a GNU extension.

     Effective: next 'PX' call.

     Default: '\w'000''

   (1) defined in *note ms Footnotes::

4.6.4 Document Description Macros
---------------------------------

Only the simplest document lacks a title.(1)  (*note ms Document
Description Macros-Footnote-1::) As its level of sophistication (or
complexity) increases, it tends to acquire a date of revision,
explicitly identified authors, sponsoring institutions for authors, and,
at the rarefied heights, an abstract of its content.  Define these data
by calling the macros below in the order shown; 'DA' or 'ND' can be
called to set the document date (or other identifier) at any time before
(a) the abstract, if present, or (b) its information is required in a
header or footer.  Use of these macros is optional, except that 'TL' is
mandatory if any of 'RP', 'AU', 'AI', or 'AB' is called, and 'AE' is
mandatory if 'AB' is called.

 -- Macro: .RP [no-repeat-info] [no-renumber]
     Use the "report" (AT&T: "released paper") format for your document,
     creating a separate cover page.  The default arrangement is to
     place most of the document description (title, author names and
     institutions, and abstract, but not the date) at the top of the
     first page.  If the optional 'no-repeat-info' argument is given,
     'ms' produces a cover page but does not repeat any of its
     information subsequently (but see the 'DA' macro below regarding
     the date).  Normally, 'RP' sets the page number following the cover
     page to 1.  Specifying the optional 'no-renumber' argument
     suppresses this alteration.  Optional arguments can occur in any
     order.  'no' is recognized as a synonym of 'no-repeat-info' for
     'AT&T' compatibility.

 -- Macro: .TL
     Specify the document title.  'ms' collects text on input lines
     following this call into the title until reaching 'AU', 'AB', or a
     heading or paragraphing macro call.

 -- Macro: .AU
     Specify an author's name.  'ms' collects text on input lines
     following this call into the author's name until reaching 'AI',
     'AB', another 'AU', or a heading or paragraphing macro call.  Call
     it repeatedly to specify multiple authors.

 -- Macro: .AI
     Specify the preceding author's institution.  An 'AU' call is
     usefully followed by at most one 'AI' call; if there are more, the
     last 'AI' call controls.  'ms' collects text on input lines
     following this call into the author's institution until reaching
     'AU', 'AB', or a heading or paragraphing macro call.

 -- Macro: .DA [x ...]
     Typeset the current date, or any arguments X, in the center footer,
     and, if 'RP' is also called, left-aligned at the end of the
     description information on the cover page.

 -- Macro: .ND [x ...]
     Typeset the current date, or any arguments X, if 'RP' is also
     called, left-aligned at the end of the document description on the
     cover page.  This is 'groff' 'ms''s default.

 -- Macro: .AB [no]
     Begin the abstract.  'ms' collects text on input lines following
     this call into the abstract until reaching an 'AE' call.  By
     default, 'ms' places the word "ABSTRACT" centered and in italics
     above the text of the abstract.  The optional argument 'no'
     suppresses this heading.

 -- Macro: .AE
     End the abstract.

   An example document description, using a cover page, follows.

     .RP
     .TL
     The Inevitability of Code Bloat
     in Commercial and Free Software
     .AU
     J.\& Random Luser
     .AI
     University of West Bumblefuzz
     .AB
     This report examines the long-term growth of the code
     bases in two large,
     popular software packages;
     the free Emacs and the commercial Microsoft Word.
     While differences appear in the type or order of
     features added,
     due to the different methodologies used,
     the results are the same in the end.
     .PP
     The free software approach is shown to be superior in
     that while free software can become as bloated as
     commercial offerings,
     free software tends to have fewer serious bugs and the
     added features are more in line with user demand.
     .AE

     ...the rest of the paper...

   (1) Distinguish a document title from "titles", which are what 'roff'
systems call headers and footers collectively.

4.6.5 Body Text
---------------

A variety of macros, registers, and strings can be used to structure and
style the body of your document.  They organize your text into
paragraphs, headings, footnotes, and inclusions of material such as
tables and figures.

4.6.5.1 Text settings
.....................

The 'FAM' string, a GNU extension, sets the font family for body text;
the default is 'T'.  The 'PS' and 'VS' registers set the type size and
vertical spacing (distance between text baselines), respectively.  The
font family and type size are ignored on terminal devices.  Setting
these parameters before the first call of a heading, paragraphing, or
(non-date) document description macro also applies them to headers,
footers, and (for 'FAM') footnotes.

   Which font families are available depends on the output device; as a
convention, 'T' selects a serif family ("Times"), 'H' a sans-serif
family ("Helvetica"), and 'C' a monospaced family ("Courier").  The man
page for the output driver documents its font repertoire.  Consult the
'groff(1)' man page for lists of available output devices and their
drivers.

   The hyphenation mode (as used by the 'hy' request) is set from the
'HY' register.  Setting 'HY' to '0' is equivalent to using the 'nh'
request.  This is a Tenth Edition Research Unix extension.

4.6.5.2 Typographical symbols
.............................

'ms' provides a few strings to obtain typographical symbols not easily
entered with the keyboard.  These and many others are available as
special character escape sequences--see the 'groff_char(7)' man page.

 -- String: \*[-]
     Interpolate an em dash.

 -- String: \*[Q]
 -- String: \*[U]
     Interpolate typographer's quotation marks where available, and
     neutral double quotes otherwise.  '\*Q' is the left quote and '\*U'
     the right.

4.6.5.3 Paragraphs
..................

Paragraphing macros "break", or terminate, any pending output line so
that a new paragraph can begin.  Several paragraph types are available,
differing in how indentation applies to them: to left, right, or both
margins; to the first output line of the paragraph, all output lines, or
all but the first.  All paragraphing macro calls cause the insertion of
vertical space in the amount stored in the 'PD' register, except at page
or column breaks.  Alternatively, a blank input line breaks the output
line and vertically spaces by one vee.

 -- Macro: .LP
     Set a paragraph without any (additional) indentation.

 -- Macro: .PP
     Set a paragraph with a first-line left indentation in the amount
     stored in the 'PI' register.

 -- Macro: .IP [marker [width]]
     Set a paragraph with a left indentation.  The optional MARKER is
     not indented and is empty by default.  It has several applications;
     see *note Lists in ms::.  WIDTH overrides the indentation amount
     stored in the 'PI' register; its default unit is 'n'.  Once
     specified, WIDTH applies to further 'IP' calls until specified
     again or a heading or different paragraphing macro is called.

 -- Macro: .QP
     Set a paragraph indented from both left and right margins by the
     amount stored in the 'QI' register.

 -- Macro: .QS
 -- Macro: .QE
     Begin ('QS') and end ('QE') a region where each paragraph is
     indented from both margins by the amount stored in the 'QI'
     register.  The text between 'QS' and 'QE' can be structured further
     by use of other paragraphing macros.

 -- Macro: .XP
     Set an "exdented" paragraph--one with a left indentation in the
     amount stored in the 'PI' register on every line _except_ the first
     (also known as a hanging indent).  This is a Berkeley extension.

   The following example illustrates the use of paragraphing macros.

     .NH 2
     Cases used in the 2001 study
     .LP
     Two software releases were considered for this report.
     .PP
     The first is commercial software;
     the second is free.
     .IP \[bu]
     Microsoft Word for Windows,
     starting with version 1.0 through the current version
     (Word 2000).
     .IP \[bu]
     GNU Emacs,
     from its first appearance as a standalone editor through
     the current version (v20).
     See [Bloggs 2002] for details.
     .QP
     Franklin's Law applied to software:
     software expands to outgrow both RAM and disk space over
     time.
     .SH
     Bibliography
     .XP
     Bloggs, Joseph R.,
     .I "Everyone's a Critic" ,
     Underground Press, March 2002.
     A definitive work that answers all questions and
     criticisms about the quality and usability of free
     software.

4.6.5.4 Headings
................

Use headings to create a sequential or hierarchical structure for your
document.  The 'ms' macros print headings in *bold* using the same font
family and, by default, type size as the body text.  Headings are
available with and without automatic numbering.  Text on input lines
following the macro call becomes the heading's title.

 -- Macro: .NH [depth]
 -- Macro: .NH S heading-depth-index ...
     Set an automatically numbered heading.

     'ms' produces a numbered heading the form A.B.C..., to any depth
     desired, with the numbering of each depth increasing automatically
     and being reset to zero when a more significant level is increased.
     "1" is the most significant or coarsest division of the document.
     Only nonzero values are output.  If DEPTH is omitted, it is taken
     to be '1'.

     If you specify DEPTH such that an ascending gap occurs relative to
     the previous 'NH' call--that is, you "skip a depth", as by '.NH 1'
     and then '.NH 3'--'groff' 'ms' emits a warning on the standard
     error stream.

     Alternatively, you can give 'NH' a first argument of 'S', followed
     by integers to number the heading depths explicitly.  Further
     automatic numbering, if used, resumes using the specified indices
     as their predecessors.  This feature is a Berkeley extension.

   An example may be illustrative.

     .NH 1
     Animalia
     .NH 2
     Arthropoda
     .NH 3
     Crustacea
     .NH 2
     Chordata
     .NH S 6 6 6
     Daimonia
     .NH 1
     Plantae

   The above results in numbering as follows; the vertical space that
normally precedes each heading is omitted.

     1.  Animalia
     1.1.  Arthropoda
     1.1.1.  Crustacea
     1.2.  Chordata
     6.6.6.  Daimonia
     7.  Plantae

 -- String: \*[SN-STYLE]
 -- String: \*[SN-DOT]
 -- String: \*[SN-NO-DOT]
 -- String: \*[SN]
     After 'NH' is called, the assigned number is made available in the
     strings 'SN-DOT' (as it appears in a printed heading with default
     formatting, followed by a terminating period) and 'SN-NO-DOT' (with
     the terminating period omitted).  These are GNU extensions.

     You can control the style used to print numbered headings by
     defining an appropriate alias for the string 'SN-STYLE'.  By
     default, 'SN-STYLE' is aliased to 'SN-DOT'.  If you prefer to omit
     the terminating period from numbers appearing in numbered headings,
     you may define the alias as follows.

          .als SN-STYLE SN-NO-DOT

     Any such change in numbering style becomes effective from the next
     use of 'NH' following redefinition of the alias for 'SN-STYLE'.
     The formatted number of the current heading is available in the
     'SN' string (a feature first documented by Berkeley), which
     facilitates its inclusion in, for example, table captions, equation
     labels, and 'XS'/'XA'/'XE' table of contents entries.

 -- Macro: .SH [depth]
     Set an unnumbered heading.

     The optional DEPTH argument is a GNU extension indicating the
     heading depth corresponding to the DEPTH argument of 'NH'.  It
     matches the type size at which the heading is set to that of a
     numbered heading at the same depth when the 'GROWPS' and 'PSINCR'
     heading size adjustment mechanism is in effect.

   If the 'GROWPS' register is set to a value greater than the LEVEL
argument to 'NH' or 'SH', the type size of a heading produced by these
macros increases by 'PSINCR' units over the size specified by 'PS'
multiplied by the difference of 'GROWPS' and LEVEL.  The value stored in
'PSINCR' is interpreted in 'groff' basic units; the 'p' scaling unit
should be employed when assigning a value specified in points.  For
example, the sequence

     .nr PS 10
     .nr GROWPS 3
     .nr PSINCR 1.5p
     .NH 1
     Carnivora
     .NH 2
     Felinae
     .NH 3
     Felis catus
     .SH 2
     Machairodontinae

will cause "1.  Carnivora" to be printed in 13-point text, followed by
"1.1.  Felinae" in 11.5-point text, while "1.1.1.  Felis catus" and all
more deeply nested heading levels will remain in the 10-point text
specified by the 'PS' register.  "Machairodontinae" is printed at 11.5
points, since it corresponds to heading level 2.

   The 'HORPHANS' register operates in conjunction with the 'NH' and
'SH' macros to inhibit the printing of isolated headings at the bottom
of a page; it specifies the minimum number of lines of an immediately
subsequent paragraph that must be kept on the same page as the heading.
If insufficient space remains on the current page to accommodate the
heading and this number of lines of paragraph text, a page break is
forced before the heading is printed.  Any display macro call or 'tbl',
'pic', or 'eqn' region between the heading and the subsequent paragraph
suppresses this grouping.  *Note ms keeps and displays:: and *note ms
Insertions::.

4.6.5.5 Typeface and decoration
...............................

The 'ms' macros provide a variety of ways to style text.  Attend closely
to the ordering of arguments labeled PRE and POST, which is not
intuitive.  Support for PRE arguments is a GNU extension.(1)  (*note
Typeface and decoration-Footnote-1::)

 -- Macro: .B [text [post [pre]]]
     Style TEXT in bold, followed by POST in the previous font style
     without intervening space, and preceded by PRE similarly.  Without
     arguments, 'ms' styles subsequent text in bold until the next
     paragraphing, heading, or no-argument typeface macro call.

 -- Macro: .R [text [post [pre]]]
     As 'B', but use the roman style (upright text of normal weight)
     instead of bold.  Argument recognition is a GNU extension.

 -- Macro: .I [text [post [pre]]]
     As 'B', but use an italic or oblique style instead of bold.

 -- Macro: .BI [text [post [pre]]]
     As 'B', but use a bold italic or bold oblique style instead of
     upright bold.  This is a Tenth Edition Research Unix extension.

 -- Macro: .CW [text [post [pre]]]
     As 'B', but use a constant-width (monospaced) roman typeface
     instead of bold.  This is a Tenth Edition Research Unix extension.

 -- Macro: .BX [text]
     Typeset TEXT and draw a box around it.  On terminal devices,
     reverse video is used instead.  If you want TEXT to contain space,
     use unbreakable space or horizontal motion escape sequences ('\~',
     '\SP', '\^', '\|', '\0' or '\h').

 -- Macro: .UL [text [post]]
     Typeset TEXT with an underline.  POST, if present, is set after
     TEXT with no intervening space.

 -- Macro: .LG
     Set subsequent text in larger type (two points larger than the
     current size) until the next type size, paragraphing, or heading
     macro call.  You can specify this macro multiple times to enlarge
     the type size as needed.

 -- Macro: .SM
     Set subsequent text in smaller type (two points smaller than the
     current size) until the next type size, paragraphing, or heading
     macro call.  You can specify this macro multiple times to reduce
     the type size as needed.

 -- Macro: .NL
     Set subsequent text at the normal type size (the amount in the 'PS'
     register).

   PRE and POST arguments are typically used to simplify the attachment
of punctuation to styled words.  When PRE is used, a hyphenation control
escape sequence '\%' that would ordinarily start TEXT must start PRE
instead to have the desired effect.

     The CS course's students found one C language keyword
     .CW static ) \%(
     most troublesome.

   The foregoing example produces output as follows.

     The CS course's students found one C language keyword (static)
     most troublesome.

   You can use the output line continuation escape sequence '\c' to
achieve the same result (*note Line Continuation::).  It is also
portable to older 'ms' implementations.

     The CS course's students found one C language keyword
     \%(\c
     .CW \%static )
     most troublesome.

   'groff' 'ms' also offers strings to begin and end super- and
subscripting.  These are GNU extensions.

 -- String: \*[{]
 -- String: \*[}]
     Begin and end superscripting, respectively.

 -- String: \*[<]
 -- String: \*[>]
     Begin and end subscripting, respectively.

   Rather than calling the 'CW' macro, in 'groff' 'ms' you might prefer
to change the font family to Courier by setting the 'FAM' string to 'C'.
You can then use all four style macros above, returning to the default
family (Times) with '.ds FAM T'.  Because changes to 'FAM' take effect
only at the next paragraph, 'CW' remains useful to "inline" a change to
the font family, similarly to the practice of this document in noting
syntactical elements of 'ms' and 'groff'.

   (1) This idiosyncrasy arose through feature accretion; for example,
the 'B' macro in Version 6 Unix 'ms' (1975) accepted only one argument,
the text to be set in boldface.  By Version 7 (1979) it recognized a
second argument; in 1990, 'groff' 'ms' added a "pre" argument, placing
it third to avoid breaking support for older documents.

4.6.5.6 Lists
.............

The MARKER argument to the 'IP' macro can be employed to present a
variety of lists; for instance, you can use a bullet glyph ('\[bu]') for
unordered lists, a number (or auto-incrementing register) for numbered
lists, or a word or phrase for glossary-style or definition lists.  If
you set the paragraph indentation register 'PI' before calling 'IP', you
can later reorder the items in the list without having to ensure that a
WIDTH argument remains affixed to the first call.

   The following is an example of a bulleted list.

     .nr PI 2n
     A bulleted list:
     .IP \[bu]
     lawyers
     .IP \[bu]
     guns
     .IP \[bu]
     money

     A bulleted list:

     * lawyers

     * guns

     * money

   The following is an example of a numbered list.

     .nr step 0 1
     .nr PI 3n
     A numbered list:
     .IP \n+[step]
     lawyers
     .IP \n+[step]
     guns
     .IP \n+[step]
     money

     A numbered list:

     1. lawyers

     2. guns

     3. money

   Here we have employed the 'nr' request to create a register of our
own, 'step'.  We initialized it to zero and assigned it an
auto-increment of 1.  Each time we use the escape sequence '\n+[PI]'
(note the plus sign), the formatter applies the increment just before
interpolating the register's value.  Preparing the 'PI' register as well
enables us to rearrange the list without the tedium of updating macro
calls.

   The next example illustrates a glossary-style list.

     A glossary-style list:
     .IP lawyers 0.4i
     Two or more attorneys.
     .IP guns
     Firearms,
     preferably large-caliber.
     .IP money
     Gotta pay for those
     lawyers and guns!

     A glossary-style list:

     lawyers
           Two or more attorneys.

     guns  Firearms, preferably large-caliber.

     money
           Gotta pay for those lawyers and guns!

   In the previous example, observe how the 'IP' macro places the
definition on the same line as the term if it has enough space.  If this
is not what you want, there are a few workarounds we will illustrate by
modifying the example.  First, you can use a 'br' request to force a
break after printing the term or label.

     .IP guns
     .br
     Firearms,

   Second, you could apply the '\p' escape sequence to force a break.
The space following the escape sequence is important; if you omit it,
'groff' prints the first word of the paragraph text on the same line as
the term or label (if it fits) _then_ breaks the line.

     .IP guns
     \p Firearms,

   Finally, you may append a horizontal motion to the marker with the
'\h' escape sequence; using the same amount as the indentation will
ensure that the marker is too wide for 'groff' to treat it as "fitting"
on the same line as the paragraph text.

     .IP guns\h'0.4i'
     Firearms,

   In each case, the result is the same.

     A glossary-style list:

     lawyers
           Two or more attorneys.

     guns
           Firearms, preferably large-caliber.

     money
           Gotta pay for those lawyers and guns!

4.6.5.7 Indented regions
........................

You may need to indent a region of text while otherwise formatting it
normally.  Indented regions can be nested; you can change '\n[PI]'
before each call to vary the amount of inset.

 -- Macro: .RS
     Begin a region where headings, paragraphs, and displays are
     indented (further) by the amount stored in the 'PI' register.

 -- Macro: .RE
     End the (next) most recent indented region.

   This feature enables you to easily line up text under hanging and
indented paragraphs.  For example, you may wish to structure lists
hierarchically.

     .IP \[bu] 2
     Lawyers:
     .RS
     .IP \[bu]
     Dewey,
     .IP \[bu]
     Cheatham,
     and
     .IP \[bu]
     and Howe.
     .RE
     .IP \[bu]
     Guns

     * Lawyers:

       *  Dewey,

       *  Cheatham, and

       *  Howe.

     * Guns

4.6.5.8 Keeps, boxed keeps, and displays
........................................

On occasion, you may want to "keep" several lines of text, or a region
of a document, together on a single page, preventing an automatic page
break within certain boundaries.  This can cause a page break to occur
earlier than it normally would.  For example, you may want to keep two
paragraphs together, or a paragraph that refers to a table, list, or
figure adjacent to the item it discusses.  'ms' provides the 'KS' and
'KE' macros for this purpose.

   You can alternatively specify a "floating keep": if a keep cannot fit
on the current page, 'ms' holds its contents and allows material
following the keep (in the source document) to fill the remainder of the
current page.  When the page breaks, whether by reaching the end or 'bp'
request, 'ms' puts the floating keep at the beginning of the next page.
This is useful for placing large graphics or tables that do not need to
appear exactly where they occur in the source document.

 -- Macro: .KS
 -- Macro: .KF
 -- Macro: .KE
     'KS' begins a keep, 'KF' a floating keep, and 'KE' ends a keep of
     either kind.

   As an alternative to the keep mechanism, the 'ne' request forces a
page break if there is not at least the amount of vertical space
specified in its argument remaining on the page (*note Page Control::).
One application of 'ne' is to reserve space on the page for a figure or
illustration to be included later.

   A "boxed keep" has a frame drawn around it.

 -- Macro: .B1
 -- Macro: .B2
     'B1' begins a keep with a box drawn around it.  'B2' ends a boxed
     keep.

   Boxed keep macros cause breaks; if you need to box a word or phrase
within a line, see the 'BX' macro in *note Typeface and decoration::.
Box lines are drawn as close as possible to the text they enclose so
that they are usable within paragraphs.  If you wish to box one or more
paragraphs, you may improve the appearance by calling 'B1' after the
first paragraphing macro, and by adding a small amount of vertical space
before calling 'B2'.

     .LP
     .B1
     .I Warning:
     Happy Fun Ball may suddenly accelerate to dangerous
     speeds.
     .sp \n[PD]/2 \" space by half the inter-paragraph distance
     .B2

   If you want a boxed keep to float, you will need to enclose the 'B1'
and 'B2' calls within a pair of 'KF' and 'KE' calls.

   "Displays" turn off filling; lines of verse or program code are shown
with their lines broken as in the source document without requiring 'br'
requests between lines.  Displays can be kept on a single page or
allowed to break across pages.  The 'DS' macro begins a kept display of
the layout specified in its first argument; non-kept displays are begun
with dedicated macros corresponding to their layout.

 -- Macro: .DS L
 -- Macro: .LD
     Begin ('DS': kept) left-aligned display.

 -- Macro: .DS [I [indent]]
 -- Macro: .ID [indent]
     Begin ('DS': kept) display indented by INDENT if specified, and by
     the amount of the 'DI' register otherwise.

 -- Macro: .DS B
 -- Macro: .BD
     Begin a ('DS': kept) a block display: the entire display is
     left-aligned, but indented such that the longest line in the
     display is centered on the page.

 -- Macro: .DS C
 -- Macro: .CD
     Begin a ('DS': kept) centered display: each line in the display is
     centered.

 -- Macro: .DS R
 -- Macro: .RD
     Begin a ('DS': kept) right-aligned display.  This is a GNU
     extension.

 -- Macro: .DE
     End any display.

   The distance stored in the 'DD' register is inserted before and after
each pair of display macros, replacing any adjacent inter-paragraph
distance; this is a Berkeley extension.  The 'DI' register is a GNU
extension; its value is an indentation applied to displays created with
'.DS' and '.ID' without arguments, to '.DS I' without an indentation
argument, and to indented equations set with '.EQ'.  Changes to either
register take effect at the next display boundary.

4.6.5.9 Tables, figures, equations, and references
..................................................

The 'ms' package is often used with the 'tbl', 'pic', 'eqn', and 'refer'
preprocessors.  Mark text meant for preprocessors by enclosing it in
pairs of tokens as follows, with nothing between the dot and the macro
name.  The preprocessors match these tokens only at the start of an
input line.

 -- Macro: .TS [H]
 -- Macro: .TE
     Demarcate a table to be processed by the 'tbl' preprocessor.  The
     optional argument 'H' to 'TS' instructs 'ms' to repeat table rows
     (often column headings) at the top of each new page the table
     spans, if applicable; calling the 'TH' macro marks the end of such
     rows.  The GNU 'tbl(1)' man page provides a comprehensive reference
     to the preprocessor and offers examples of its use.

 -- Macro: .PS
 -- Macro: .PE
 -- Macro: .PF
     'PS' begins a picture to be processed by the 'gpic' preprocessor;
     either of 'PE' or 'PF' ends it, the latter with "flyback" to the
     vertical position at its top.  You can create 'pic' input manually
     or with a program such as 'xfig'.

 -- Macro: .EQ [align [label]]
 -- Macro: .EN
     Demarcate an equation to be processed by the 'eqn' preprocessor.
     The equation is centered by default; ALIGN can be 'C', 'L', or 'I'
     to (explicitly) center, left-align, or indent it by the amount
     stored in the 'DI' register, respectively.  If specified, LABEL is
     set right-aligned.

 -- Macro: .[
 -- Macro: .]
     Demarcate a bibliographic citation to be processed by the 'refer'
     preprocessor.  The GNU 'refer(1)' man page provides a comprehensive
     reference to the preprocessor and the format of its bibliographic
     database.  Type 'man refer' at the command line to view it.

   When 'refer' emits collected references (as might be done on a "Works
Cited" page), it interpolates the 'REFERENCES' string as an unnumbered
heading ('SH').

   The following is an example of how to set up a table that may print
across two or more pages.

     .TS H
     allbox;
     Cb | Cb .
     Part->Description
     _
     .TH
     .T&
     GH-1978->Fribulating gonkulator
     ...the rest of the table follows...
     .TE

Attempting to place a multi-page table inside a keep can lead to
unpleasant results, particularly if the 'tbl' 'allbox' option is used.

   Mathematics can be typeset using the language of the 'eqn'
preprocessor.

     .EQ C (\*[SN-NO-DOT]a)
     p ~ = ~ q sqrt { ( 1 + ~ ( x / q sup 2 ) }
     .EN

This input formats a labelled equation.  We used the 'SN-NO-DOT' string
to base the equation label on the current heading number, giving us more
flexibility to reorganize the document.

   Use 'groff' options to run preprocessors on the input: '-e' for
'geqn', '-p' for 'gpic', '-R' for 'grefer', and '-t' for 'gtbl'.

4.6.5.10 Footnotes
..................

A footnote is typically anchored to a place in the text with a "marker",
which is a small integer, a symbol such as a dagger, or arbitrary
user-specified text.

 -- String: \*[*]
     Place an "automatic number", an automatically generated numeric
     footnote marker, in the text.  Each time this string is
     interpolated, the number it produces increments by one.  Automatic
     numbers start at 1.  This is a Berkeley extension.

   Enclose the footnote text in 'FS' and 'FE' macro calls to set it at
the nearest available "foot", or bottom, of a text column or page.

 -- Macro: .FS [marker]
 -- Macro: .FE
     Begin ('FS') and end ('FE') a footnote.  'FS' calls 'FS-MARK' with
     any supplied MARKER argument, which is then also placed at the
     beginning of the footnote text.  If MARKER is omitted, the next
     pending automatic footnote number enqueued by interpolation of the
     '*' string is used, and if none exists, nothing is prefixed.

   You may not desire automatically numbered footnotes in spite of their
convenience.  You can indicate a footnote with a symbol or other text by
specifying its marker at the appropriate place (for example, by using
'\[dg]' for the dagger glyph) _and_ as an argument to the 'FS' macro.
Such manual marks should be repeated as arguments to 'FS' or as part of
the footnote text to disambiguate their correspondence.  You may wish to
use '\*{' and '\*}' to superscript the marker at the anchor point, in
the footnote text, or both.

   'groff' 'ms' provides a hook macro, 'FS-MARK', for user-determined
operations to be performed when the 'FS' macro is called.  It is passed
the same arguments as 'FS' itself.  An application of 'FS-MARK' is
anchor placement for a hyperlink reference, so that a footnote can link
back to its referential context.(1)  (*note ms Footnotes-Footnote-1::)
By default, this macro has an empty definition.  'FS-MARK' is a GNU
extension.

   Footnotes can be safely used within keeps and displays, but you
should avoid using automatically numbered footnotes within floating
keeps.  You can place a second '\**' interpolation between a '\**' and
its corresponding 'FS' call as long as each 'FS' call occurs _after_ the
corresponding '\**' and occurrences of 'FS' are in the same order as
corresponding occurrences of '\**'.

   Footnote text is formatted as paragraphs are, using analogous
parameters.  The registers 'FI', 'FPD', 'FPS', and 'FVS' correspond to
'PI', 'PD', 'PS', and 'CS', respectively; 'FPD', 'FPS', and 'FVS' are
GNU extensions.

   The 'FF' register controls the formatting of automatically numbered
footnote paragraphs and those for which 'FS' is given a marker argument.
*Note ms Document Control Settings::.

   The default footnote line length is 11/12ths of the normal line
length for compatibility with the expectations of historical 'ms'
documents; you may wish to set the 'FR' string to '1' to align with
contemporary typesetting practices.  In the past,(2) (*note ms
Footnotes-Footnote-2::) an 'FL' register was used for the line length in
footnotes; however, setting this register at document initialization
time had no effect on the footnote line length in multi-column
arrangements.(3)  (*note ms Footnotes-Footnote-3::)

   'FR' should be used in preference to the old 'FL' register in
contemporary documents.  The footnote line length is effectively
computed as 'column-width * \*[FR]'.  If an absolute footnote line
length is required, recall that arithmetic expressions in 'roff' input
are evaluated strictly from left to right, with no operator precedence
(parentheses are honored).

     .ds FR 0+3i \" Set footnote line length to 3 inches.

   (1) "Portable Document Format Publishing with GNU Troff",
'pdfmark.ms' in the 'groff' distribution, uses this technique.

   (2) Unix Version 7 'ms', its descendants, and GNU 'ms' prior to
'groff' version 1.23.0

   (3) You could reset it after each call to '.1C', '.2C', or '.MC'.

4.6.5.11 Language and localization
..................................

'groff' 'ms' provides several strings that you can customize for your
own purposes, or redefine to adapt the macro package to languages other
than English.  It is already localized for Czech, German, French,
Italian, and Swedish.  Load the desired localization macro package after
'ms'; see the 'groff_tmac(5)' man page.

     $ groff -ms -mfr bienvenue.ms

   The following strings are available.

 -- String: \*[REFERENCES]
     Contains the string printed at the beginning of a references
     (bibliography) page produced with GNU 'refer(1)'.  The default is
     'References'.

 -- String: \*[ABSTRACT]
     Contains the string printed at the beginning of the abstract.  The
     default is '\f[I]ABSTRACT\f[]'; it includes font selection escape
     sequences to set the word in italics.

 -- String: \*[TOC]
     Contains the string printed at the beginning of the table of
     contents.  The default is 'Table of Contents'.

 -- String: \*[MONTH1]
 -- String: \*[MONTH2]
 -- String: \*[MONTH3]
 -- String: \*[MONTH4]
 -- String: \*[MONTH5]
 -- String: \*[MONTH6]
 -- String: \*[MONTH7]
 -- String: \*[MONTH8]
 -- String: \*[MONTH9]
 -- String: \*[MONTH10]
 -- String: \*[MONTH11]
 -- String: \*[MONTH12]
     Contain the full names of the calendar months.  The defaults are in
     English: 'January', 'February', and so on.

4.6.6 Page layout
-----------------

'ms''s default page layout arranges text in a single column with the
page number between hyphens centered in a header on each page except the
first, and produces no footers.  You can customize this arrangement.

4.6.6.1 Headers and footers
...........................

There are multiple ways to produce headers and footers.  One is to
define the strings 'LH', 'CH', and 'RH' to set the left, center, and
right headers, respectively; and 'LF', 'CF', and 'RF' to set the left,
center, and right footers.  This approach suffices for documents that do
not distinguish odd- and even-numbered pages.

   Another method is to call macros that set headers or footers for odd-
or even-numbered pages.  Each such macro takes a delimited argument
separating the left, center, and right header or footer texts from each
other.  You can replace the neutral apostrophes (''') shown below with
any character not appearing in the header or footer text.  These macros
are Berkeley extensions.

 -- Macro: .OH 'left'center'right'
 -- Macro: .EH 'left'center'right'
 -- Macro: .OF 'left'center'right'
 -- Macro: .EF 'left'center'right'
     The 'OH' and 'EH' macros define headers for odd- (recto) and
     even-numbered (verso) pages, respectively; the 'OF' and 'EF' macros
     define footers for them.

   With either method, a percent sign '%' in header or footer text is
replaced by the current page number.  By default, 'ms' places no header
on a page numbered "1" (regardless of its number format).

 -- Macro: .P1
     Typeset the header even on page 1.  To be effective, this macro
     must be called before the header trap is sprung on any page
     numbered "1"; in practice, unless your page numbering is unusual,
     this means that you should call it early, before 'TL' or any
     heading or paragraphing macro.  This is a Berkeley extension.

   For even greater flexibility, 'ms' is designed to permit the
redefinition of the macros that are called when the 'groff' traps that
ordinarily cause the headers and footers to be output are sprung.  'PT'
("page trap") is called by 'ms' when the header is to be written, and
'BT' ("bottom trap") when the footer is to be.  The 'groff' page
location trap that 'ms' sets up to format the header also calls the
(normally undefined) 'HD' macro after 'PT'; you can define 'HD' if you
need additional processing after setting the header (for example, to
draw a line below it).  The 'HD' hook is a Berkeley extension.  Any such
macros you (re)define must implement any desired specialization for
odd-, even-, or first numbered pages.

4.6.6.2 Tab stops
.................

Use the 'ta' request to define tab stops as needed.  *Note Tabs and
Fields::.

 -- Macro: .TA
     Reset the tab stops to the 'ms' default (every 5 ens).  Redefine
     this macro to create a different set of default tab stops.

4.6.6.3 Margins
...............

Control margins using the registers summarized in "Margin settings" in
*note ms Document Control Settings:: above.  There is no setting for the
right margin; the combination of page offset '\n[PO]' and line length
'\n[LL]' determines it.

4.6.6.4 Multiple columns
........................

'ms' can set text in as many columns as reasonably fit on the page.  The
following macros force a page break if a multi-column layout is active
when they are called.  The 'MINGW' register stores the default minimum
gutter width; it is a GNU extension.  When multiple columns are in use,
keeps and the 'HORPHANS' and 'PORPHANS' registers work with respect to
column breaks instead of page breaks.

 -- Macro: .1C
     Arrange page text in a single column (the default).

 -- Macro: .2C
     Arrange page text in two columns.

 -- Macro: .MC [column-width [gutter-width]]
     Arrange page text in multiple columns.  If you specify no
     arguments, it is equivalent to the '2C' macro.  Otherwise,
     COLUMN-WIDTH is the width of each column and GUTTER-WIDTH is the
     minimum distance between columns.

4.6.6.5 Creating a table of contents
....................................

Because 'roff' formatters process their input in a single pass, material
on page 50, for example, cannot influence what appears on page 1--this
poses a challenge for a table of contents at its traditional location in
front matter, if you wish to avoid manually maintaining it.  'ms'
enables the collection of material to be presented in the table of
contents as it appears, saving its page number along with it, and then
emitting the collected contents on demand toward the end of the
document.  The table of contents can then be resequenced to its desired
location as part of post-processing--with a PDF page relocation tool, or
by physically rearranging the pages of a printed document, depending on
the output format and circumstances.

   Define an entry to appear in the table of contents by bracketing its
text between calls to the 'XS' and 'XE' macros.  A typical application
is to call them immediately after 'NH' or 'SH' and repeat the heading
text within them.  The 'XA' macro, used within '.XS'/'.XE' pairs,
supplements an entry--for instance, when it requires multiple output
lines, whether because a heading is too long to fit or because style
dictates that page numbers not be repeated.  You may wish to indent the
text thus wrapped to correspond to its heading depth; this can be done
in the entry text by prefixing it with tabs or horizontal motion escape
sequences, or by providing a second argument to the 'XA' macro.  'XS'
and 'XA' automatically associate the page number where they are called
with the text following them, but they accept arguments to override this
behavior.  At the end of the document, call 'TC' or 'PX' to emit the
table of contents; 'TC' resets the page number to 'i' (Roman numeral
one), and then calls 'PX'.  All of these macros are Berkeley extensions.

 -- Macro: .XS [page-number]
 -- Macro: .XA [page-number [indentation]]
 -- Macro: .XE
     Begin, supplement, and end a table of contents entry.  Each entry
     is associated with PAGE-NUMBER (otherwise the current page number);
     a PAGE-NUMBER of 'no' prevents a leader and page number from being
     emitted for that entry.  Use of 'XA' within 'XS'/'XE' is optional;
     it can be repeated.  If INDENTATION is present, a supplemental
     entry is indented by that amount; ens are assumed if no unit is
     indicated.  Text on input lines between 'XS' and 'XE' is stored for
     later recall by 'PX'.

 -- Macro: .PX [no]
     Switch to single-column layout.  Unless 'no' is specified, center
     and interpolate the 'TOC' string in bold and two points larger than
     the body text.  Emit the table of contents entries.

 -- Macro: .TC [no]
     Set the page number to 1, the page number format to lowercase Roman
     numerals, and call 'PX' (with a 'no' argument, if present).

   Here's an example of typical 'ms' table of contents preparation.  We
employ horizontal escape sequences '\h' to indent the entries by
sectioning depth.

     .NH 1
     Introduction
     .XS
     Introduction
     .XE
     ...
     .NH 2
     Methodology
     .XS
     \h'2n'Methodology
     .XA
     \h'4n'Fassbinder's Approach
     \h'4n'Kahiu's Approach
     .XE
     ...
     .NH 1
     Findings
     .XS
     Findings
     .XE
     ...
     .TC

   The remaining features in this subsubsection are GNU extensions.
'groff' 'ms' obviates the need to repeat heading text after 'XS' calls.
Call 'XN' and 'XH' after 'NH' and 'SH', respectively.

 -- Macro: .XN heading-text
 -- Macro: .XH depth heading-text
     Format HEADING-TEXT and create a corresponding table of contents
     entry.  'XN' computes the indentation from the depth of the
     preceding 'NH' call; 'XH' requires a DEPTH argument to do so.

   'groff' 'ms' encourages customization of table of contents entry
production.

 -- Macro: .XN-REPLACEMENT heading-text
 -- Macro: .XH-REPLACEMENT depth heading-text
     These hook macros implement 'XN' and 'XH', respectively.  They call
     'XN-INIT' and pass their HEADING-TEXT arguments to 'XH-UPDATE-TOC'.

 -- Macro: .XN-INIT
 -- Macro: .XH-UPDATE-TOC depth heading-text
     The 'XN-INIT' hook macro does nothing by default.  'XH-UPDATE-TOC'
     brackets HEADING-TEXT with 'XS' and 'XE' calls, indenting it by 2
     ens per level of DEPTH beyond the first.

   We could therefore produce a table of contents similar to that in the
previous example with fewer macro calls.  (The difference is that this
input follows the "Approach" entries with leaders and page numbers.)

     .NH 1
     .XN Introduction
     ...
     .NH 2
     .XN Methodology
     .XH 3 "Fassbinder's Approach"
     .XH 3 "Kahiu's Approach"
     ...
     .NH 1
     .XN Findings
     ...

   To get the section number of the numbered headings into the table of
contents entries, we might define 'XN-REPLACEMENT' as follows.  (We
obtain the heading depth from 'groff' 'ms''s internal register 'nh*hl'.)

     .de XN-REPLACEMENT
     .XN-INIT
     .XH-UPDATE-TOC \\n[nh*hl] \\$@
     \&\\*[SN] \\$*
     ..

   You can change the style of the leader that bridges each table of
contents entry with its page number; define the 'TC-LEADER' special
character by using the 'char' request.  A typical leader combines the
dot glyph '.' with a horizontal motion escape sequence to spread the
dots.  The width of the page number field is stored in the 'TC-MARGIN'
register.

4.6.7 Differences from AT&T 'ms'
--------------------------------

The 'groff' 'ms' macros are an independent reimplementation, using no
AT&T code.  Since they take advantage of the extended features of
'groff', they cannot be used with AT&T 'troff'.  'groff' 'ms' supports
several features described above as Berkeley and Tenth Edition Research
Unix extensions, and adds several of its own.

   * The internals of 'groff' 'ms' differ from the internals of AT&T
     'ms'.  Documents that depend upon implementation details of AT&T
     'ms' may not format properly with 'groff' 'ms'.  Such details
     include macros whose function was not documented in the AT&T 'ms'
     manual.(1)  (*note Differences from AT&T ms-Footnote-1::)

   * The error-handling policy of 'groff' 'ms' is to detect and report
     errors, rather than silently to ignore them.

   * Tenth Edition Research Unix supported 'P1'/'P2' macros to bracket
     code examples; 'groff' 'ms' does not.

   * 'groff' 'ms' does not work in GNU 'troff''s AT&T compatibility
     mode.  If loaded when that mode is enabled, it aborts processing
     with a diagnostic message.

   * Multiple line spacing is not supported.  Use a larger vertical
     spacing instead.

   * 'groff' 'ms' uses the same header and footer defaults in both
     'nroff' and 'troff' modes as AT&T 'ms' does in 'troff' mode; AT&T's
     default in 'nroff' mode is to put the date, in U.S. traditional
     format (e.g., "January 1, 2021"), in the center footer (the 'CF'
     string).

   * Many 'groff' 'ms' macros, including those for paragraphs, headings,
     and displays, cause a reset of paragraph rendering parameters, and
     may change the indentation; they do so not by incrementing or
     decrementing it, but by setting it absolutely.  This can cause
     problems for documents that define additional macros of their own
     that try to manipulate indentation.  Use the 'ms' 'RS' and 'RE'
     macros instead of the 'in' request.

   * AT&T 'ms' interpreted the values of the registers 'PS' and 'VS' in
     points, and did not support the use of scaling units with them.
     'groff' 'ms' interprets values of the registers 'PS', 'VS', 'FPS',
     and 'FVS' equal to or larger than 1,000 (one thousand) as decimal
     fractions multiplied by 1,000.(2)  (*note Differences from AT&T
     ms-Footnote-2::) This threshold makes use of a scaling unit with
     these parameters practical for high-resolution devices while
     preserving backward compatibility.  It also permits expression of
     non-integral type sizes.  For example, 'groff -rPS=10.5p' at the
     shell prompt is equivalent to placing '.nr PS 10.5p' at the
     beginning of the document.

   * AT&T 'ms''s 'AU' macro supported arguments used with some document
     types; 'groff' 'ms' does not.

   * Right-aligned displays are available.  The AT&T 'ms' manual
     observes that "it is tempting to assume that '.DS R' will right
     adjust lines, but it doesn't work".  In 'groff' 'ms', it does.

   * To make 'groff' 'ms' use the default page offset (which also
     specifies the left margin), the 'PO' register must stay undefined
     until the first 'ms' macro is called.

     This implies that '\n[PO]' should not be used early in the
     document, unless it is changed also: accessing an undefined
     register automatically defines it.

   * 'groff' 'ms' supports the 'PN' register, but it is not necessary;
     you can access the page number via the usual '%' register and
     invoke the 'af' request to assign a different format to it if
     desired.(3)  (*note Differences from AT&T ms-Footnote-3::)

   * The AT&T 'ms' manual documents registers 'CW' and 'GW' as setting
     the default column width and "intercolumn gap", respectively, and
     which applied when 'MC' was called with fewer than two arguments.
     'groff' 'ms' instead treats 'MC' without arguments as synonymous
     with '2C'; there is thus no occasion for a default column width
     register.  Further, the 'MINGW' register and the second argument to
     'MC' specify a _minimum_ space between columns, not the fixed
     gutter width of AT&T 'ms'.

   * The AT&T 'ms' manual did not document the 'QI' register; Berkeley
     and 'groff' 'ms' do.

 -- Register: \n[GS]
     The register 'GS' is set to 1 by the 'groff' 'ms' macros, but is
     not used by the AT&T 'ms' package.  Documents that need to
     determine whether they are being formatted with 'groff' 'ms' or
     another implementation should test this register.

   (1) 'Typing Documents on the UNIX System: Using the -ms Macros with
Troff and Nroff', M. E. Lesk, Bell Laboratories, 1978

   (2) Register values are converted to and stored as basic units.
*Note Measurements::.

   (3) If you redefine the 'ms' 'PT' macro and desire special treatment
of certain page numbers (like '1'), you may need to handle a non-Arabic
page number format, as 'groff' 'ms''s 'PT' does; see the macro package
source.  'groff' 'ms' aliases the 'PN' register to '%'.

4.6.7.1 Unix Version 7 'ms' macros not implemented by 'groff' 'ms'
..................................................................

Several macros described in the Unix Version 7 'ms' documentation are
unimplemented by 'groff' 'ms' because they are specific to the
requirements of documents produced internally by Bell Laboratories, some
of which also require a glyph for the Bell System logo that 'groff' does
not support.  These macros implemented several document type formats
('EG', 'IM', 'MF', 'MR', 'TM', 'TR'), were meaningful only in
conjunction with the use of certain document types ('AT', 'CS', 'CT',
'OK', 'SG'), stored the postal addresses of Bell Labs sites ('HO', 'IH',
'MH', 'PY', 'WH'), or lacked a stable definition over time ('UX').  To
compatibly render historical 'ms' documents using these macros, we
advise your documents to invoke the 'rm' request to remove any such
macros it uses and then define replacements with an authentically
typeset original at hand.(1)  (*note Missing Unix Version 7 ms
Macros-Footnote-1::) For informal purposes, a simple definition of 'UX'
should maintain the readability of the document's substance.

     .rm UX
     .ds UX Unix\"

   (1) The removal beforehand is necessary because 'groff' 'ms' aliases
these macros to a diagnostic macro, and you want to redefine the aliased
name, not its target.

4.6.8 Legacy Features
---------------------

'groff' 'ms' retains some legacy features solely to support formatting
of historical documents; contemporary ones should not use them because
they can render poorly.  See the 'groff_char(7)' man page.

AT&T accent mark strings
........................

AT&T 'ms' defined accent mark strings as follows.

 -- String: \*[']
     Apply acute accent to subsequent glyph.

 -- String: \*[`]
     Apply grave accent to subsequent glyph.

 -- String: \*[:]
     Apply dieresis (umlaut) to subsequent glyph.

 -- String: \*[^]
     Apply circumflex accent to subsequent glyph.

 -- String: \*[~]
     Apply tilde accent to subsequent glyph.

 -- String: \*[C]
     Apply caron to subsequent glyph.

 -- String: \*[,]
     Apply cedilla to subsequent glyph.

Berkeley accent mark and glyph strings
......................................

Berkeley 'ms' offered an 'AM' macro; calling it redefined the AT&T
accent mark strings (except for '\*C'), applied them to the _preceding_
glyph, and defined additional strings, some for spacing glyphs.

 -- Macro: .AM
     Enable alternative accent mark and glyph-producing strings.

 -- String: \*[']
     Apply acute accent to preceding glyph.

 -- String: \*[`]
     Apply grave accent to preceding glyph.

 -- String: \*[:]
     Apply dieresis (umlaut) to preceding glyph.

 -- String: \*[^]
     Apply circumflex accent to preceding glyph.

 -- String: \*[~]
     Apply tilde accent to preceding glyph.

 -- String: \*[,]
     Apply cedilla to preceding glyph.

 -- String: \*[/]
     Apply stroke (slash) to preceding glyph.

 -- String: \*[v]
     Apply caron to preceding glyph.

 -- String: \*[_]
     Apply macron to preceding glyph.

 -- String: \*[.]
     Apply underdot to preceding glyph.

 -- String: \*[o]
     Apply ring accent to preceding glyph.

 -- String: \*[?]
     Interpolate inverted question mark.

 -- String: \*[!]
     Interpolate inverted exclamation mark.

 -- String: \*[8]
     Interpolate small letter sharp s.

 -- String: \*[q]
     Interpolate small letter o with hook accent (ogonek).

 -- String: \*[3]
     Interpolate small letter yogh.

 -- String: \*[d-]
     Interpolate small letter eth.

 -- String: \*[D-]
     Interpolate capital letter eth.

 -- String: \*[th]
     Interpolate small letter thorn.

 -- String: \*[Th]
     Interpolate capital letter thorn.

 -- String: \*[ae]
     Interpolate small  ligature.

 -- String: \*[Ae]
     Interpolate capital  ligature.

 -- String: \*[oe]
     Interpolate small oe ligature.

 -- String: \*[OE]
     Interpolate capital OE ligature.

4.6.9 Naming Conventions
------------------------

The following conventions are used for names of macros, strings, and
registers.  External names available to documents that use the 'groff'
'ms' macros contain only uppercase letters and digits.

   Internally the macros are divided into modules.  The naming
conventions are as follows.

   * Names used only within one module are of the form MODULE'*'NAME.

   * Names used outside the module in which they are defined are of the
     form MODULE'@'NAME.

   * Names associated with a particular environment are of the form
     ENVIRONMENT':'NAME; these are used only within the 'par' module.

   * NAME does not have a module prefix.

   * Constructed names used to implement arrays are of the form
     ARRAY'!'INDEX.

   Thus the 'groff' 'ms' macros reserve the following names.

   * Names containing the characters '*', '@', and ':'.

   * Names containing only uppercase letters and digits.

5 GNU 'troff' Reference
***********************

This chapter covers _all_ of the facilities of the GNU 'troff'
formatting engine.  Users of macro packages may skip it if not
interested in details.

5.1 Text
========

AT&T 'troff' was designed to take input as it would be composed on a
typewriter, including the teletypewriters used as early computer
terminals, and relieve the user drafting a document of concern with
details like line length, hyphenation breaking, and the achievement of
straight margins.  Early in its development, the program gained the
ability to prepare output for a phototypesetter; a document could then
be prepared for output to either a teletypewriter, a phototypesetter, or
both.  GNU 'troff' continues this tradition of permitting an author to
compose a single master version of a document which can then be rendered
for a variety of output formats or devices.

   'roff' input files contain text interspersed with instructions to
control the formatter.  Even in the absence of such instructions, GNU
'troff' still processes its input in several ways, by filling,
hyphenating, breaking, and adjusting it, and supplementing it with
inter-sentence space.

5.1.1 Filling
-------------

When GNU 'troff' starts up, it obtains information about the device for
which it is preparing output.(1)  (*note Filling-Footnote-1::) An
essential property is the length of the output line, such as "6.5
inches".

   GNU 'troff' interprets plain text files employing the Unix
line-ending convention.  It reads input a character at a time,
collecting words as it goes, and fits as many words together on an
output line as it can--this is known as "filling".  To GNU 'troff', a
"word" is any sequence of one or more characters that aren't spaces,
tabs, or newlines.  The exceptions separate words.(2)  (*note
Filling-Footnote-2::) To disable filling, see *note Manipulating Filling
and Adjustment::.

     It is a truth universally acknowledged
     that a single man in possession of a
     good fortune must be in want of a wife.
         => It is a truth universally acknowledged that a
         => single man in possession of a good fortune must
         => be in want of a wife.

   (1) *Note Device and Font Description Files::.

   (2) There are also _escape sequences_ which can function as word
characters, word separators, or neither--the last simply have no effect
on GNU 'troff''s idea of whether an input character is within a word or
not.

5.1.2 Sentences
---------------

A passionate debate has raged for decades among writers of the English
language over whether more space should appear between adjacent
sentences than between words within a sentence, and if so, how much, and
what other circumstances should influence this spacing.(1)  (*note
Sentences-Footnote-1::) GNU 'troff' follows the example of AT&T 'troff';
it attempts to detect the boundaries between sentences, and supplies
additional inter-sentence space between them.

     Hello, world!
     Welcome to groff.
         => Hello, world!  Welcome to groff.

   GNU 'troff' does this by flagging certain characters (normally '!',
'?', and '.') as potentially ending a sentence.  When GNU 'troff'
encounters one of these "end-of-sentence characters" at the end of a
line, or one of them is followed by two spaces on the same input line,
it appends an inter-word space followed by an inter-sentence space in
the formatted output.

     R. Harper subscribes to a maxim of P. T. Barnum.
         => R. Harper subscribes to a maxim of P. T. Barnum.

   In the above example, inter-sentence space is not added after 'P.' or
'T.' because the periods do not occur at the end of an input line, nor
are they followed by two or more spaces.  Let's imagine that we've heard
something about defamation from Mr. Harper's attorney, recast the
sentence, and reflowed it in our text editor.

     I submit that R. Harper subscribes to a maxim of P. T.
     Barnum.
         => I submit that R. Harper subscribes to a maxim of
         => P. T.  Barnum.

   "Barnum" doesn't begin a sentence!  What to do?  Let us meet our
first "escape sequence", a series of input characters that give
instructions to GNU 'troff' instead of being used to construct output
device glyphs.(2)  (*note Sentences-Footnote-2::) An escape sequence
begins with the backslash character '\' by default, an uncommon
character in natural language text, and is _always_ followed by at least
one other character, hence the term "sequence".

   The dummy character escape sequence '\&' can be used after an
end-of-sentence character to defeat end-of-sentence detection on a
per-instance basis.  We can therefore rewrite our input more
defensively.

     I submit that R.\& Harper subscribes to a maxim of P.\&
     T.\& Barnum.
         => I submit that R. Harper subscribes to a maxim of
         => P. T. Barnum.

   Adding text caused our input to wrap; now, we don't need '\&' after
'T.' but we do after 'P.'.  Consistent use of the escape sequence
ensures that potential sentence boundaries are robust to editing
activities.  Further advice along these lines will follow in *note Input
Conventions::.

   Normally, the occurrence of a visible non-end-of-sentence character
(as opposed to a space or tab) immediately after an end-of-sentence
character cancels detection of the end of a sentence.  For example, it
would be incorrect for GNU 'troff' to infer the end of a sentence after
the dot in '3.14159'.  However, several characters are treated
_transparently_ after the occurrence of an end-of-sentence character.
That is, GNU 'troff' does not cancel end-of-sentence detection when it
processes them.  This is because such characters are often used as
footnote markers or to close quotations and parentheticals.  The default
set is '"', ''', ')', ']', '*', '\[dg]', '\[dd]', '\[rq]', and '\[cq]'.
The last four are examples of "special characters", escape sequences
whose purpose is to obtain glyphs that are not easily typed at the
keyboard, or which have special meaning to GNU 'troff' (like '\'
itself).(3)  (*note Sentences-Footnote-3::)

     \[lq]The idea that the poor should have leisure has always
     been shocking to the rich.\[rq]
     (Bertrand Russell, 1935)
         => "The idea that the poor should have
         => leisure has always been shocking to
         => the rich."  (Bertrand Russell, 1935)

   The sets of characters that potentially end sentences or are
transparent to sentence endings are configurable.  See the 'cflags'
request in *note Using Symbols::.  To change the additional
inter-sentence space amount--even to remove it entirely--see *note
Manipulating Filling and Adjustment::.

   (1) A well-researched jeremiad appreciated by 'groff' contributors on
both sides of the sentence-spacing debate can be found at
<https://web.archive.org/web/20171217060354/http://www.heracliteanriver.com/?p=324>.

   (2) This statement oversimplifes; there are escape sequences whose
purpose is precisely to produce glyphs on the output device, and input
characters that _aren't_ part of escape sequences can undergo a great
deal of processing before getting to the output.

   (3) The mnemonics for the special characters shown here are "dagger",
"double dagger", "right (double) quote", and "closing (single) quote".
See the 'groff_char(7)' man page.

5.1.3 Hyphenation
-----------------

When an output line is nearly full, it is uncommon for the next word
collected from the input to exactly fill it--typically, there is room
left over only for part of the next word.  The process of splitting a
word so that it appears partially on one line (with a hyphen to indicate
to the reader that the word has been broken) with its remainder on the
next is "hyphenation".  Hyphenation points can be manually specified;
GNU 'troff' also uses a hyphenation algorithm and language-specific
pattern files (based on those used in TeX) to decide which words can be
hyphenated and where.

   Hyphenation does not always occur even when the hyphenation rules for
a word allow it; it can be disabled, and when not disabled there are
several parameters that can prevent it in certain circumstances.  *Note
Manipulating Hyphenation::.

5.1.4 Breaking
--------------

Once an output line has been filled, whether or not hyphenation has
occurred on that line, the next word read from the input will be placed
on a different output line; this is called a "break".  In this manual
and in 'roff' discussions generally, a "break" if not further qualified
always refers to the termination of an output line.  When the formatter
is filling text, it introduces breaks automatically to keep output lines
from exceeding the configured line length.  After an automatic break,
GNU 'troff' adjusts the line if applicable (see below), and then resumes
collecting and filling text on the next output line.

   Sometimes, a line cannot be broken automatically.  This usually does
not happen with natural language text unless the output line length has
been manipulated to be extremely short, but it can with specialized text
like program source code.  We can use 'perl' at the shell prompt to
contrive an example of failure to break the line.  We also employ the
'-z' option to suppress normal output.

     $ perl -e 'print "#" x 80, "\n";' | nroff -z
         error-> warning: cannot break line

   The remedy for these cases is to tell GNU 'troff' where the line may
be broken without hyphens.  This is done with the non-printing break
point escape sequence '\:'; see *note Manipulating Hyphenation::.

   What if the document author wants to stop filling lines temporarily,
for instance to start a new paragraph?  There are several solutions.  A
blank input line not only causes a break, but by default it also outputs
a one-line vertical space (effectively a blank output line).  This
behavior can be modified; see *note Blank Line Traps::.  Macro packages
may discourage or disable the blank line method of paragraphing in favor
of their own macros.

   A line that begins with one or more spaces causes a break.  The
spaces are output at the beginning of the next line without being
_adjusted_ (see below); however, this behavior can be modified (*note
Leading Space Traps::).  Again, macro packages may provide other methods
of producing indented paragraphs.  Trailing spaces on text lines are
discarded.(1)  (*note Breaking-Footnote-1::)

   What if the file ends before enough words have been collected to fill
an output line?  Or the output line is exactly full but not yet broken,
and there is no more input?  GNU 'troff' interprets the end of input as
a break.  Certain requests also cause breaks, implicitly or explicitly.
This is discussed in *note Manipulating Filling and Adjustment::.

   (1) "Text lines" are defined in *note Requests and Macros::.

5.1.5 Adjustment
----------------

After GNU 'troff' performs an automatic break, it may then "adjust" the
line, widening inter-word spaces until the text reaches the right
margin.  Extra spaces between words are preserved.  Leading and trailing
spaces are handled as noted above.  Text can be aligned to the left or
right margin only, or centered; see *note Manipulating Filling and
Adjustment::.

5.1.6 Tabs and Leaders
----------------------

GNU 'troff' translates input horizontal tab characters ("tabs") and
<Control+A> characters ("leaders") into movements to the next tab stop.
Tabs simply move to the next tab stop; leaders place enough periods to
fill the space.  Tab stops are by default located every half inch
measured from the drawing position corresponding to the beginning of the
input line; see *note Page Geometry::.  Tabs and leaders do not cause
breaks and therefore do not interrupt filling.  Below, we use arrows ->
and bullets * to indicate input tabs and leaders, respectively.

     1
     -> 2 -> 3 * 4
     -> * 5
     => 1         2       3.......4         ........5

   Tabs and leaders lend themselves to table construction.(1)  (*note
Tabs and Leaders-Footnote-1::) The tab and leader glyphs can be
configured, and further facilities for sophisticated table composition
are available; see *note Tabs and Fields::.  There are many details to
track when using such low-level features, so most users turn to the
'tbl(1)' preprocessor for table construction.

   (1) "Tab" is short for "tabulation", revealing the term's origin as a
spacing mechanism for table arrangement.

5.1.7 Requests and Macros
-------------------------

We have now encountered almost all of the syntax there is in the 'roff'
language, with an exception already noted in passing.  A "request" is an
instruction to the formatter that occurs after a "control character",
which is recognized at the beginning of an input line.  The regular
control character is a dot ('.').  Its counterpart, the "no-break
control character", a neutral apostrophe ('''), suppresses the break
that is implied by some requests.  These characters were chosen because
it is uncommon for lines of text in natural languages to begin with
them.  If you require a formatted period or apostrophe (closing single
quotation mark) where GNU 'troff' is expecting a control character,
prefix the dot or neutral apostrophe with the dummy character escape
sequence, '\&'.

   An input line beginning with a control character is called a "control
line".  Every line of input that is not a control line is a "text
line".(1)  (*note Requests and Macros-Footnote-1::)

   Requests often take "arguments", words (separated from the request
name and each other by spaces) that specify details of the action GNU
'troff' is expected to perform.  If a request is meaningless without
arguments, it is typically ignored.

   GNU 'troff''s requests and escape sequences comprise the control
language of the formatter.  Of key importance are the requests that
define macros.  Macros are invoked like requests, enabling the request
repertoire to be extended or overridden.(2)  (*note Requests and
Macros-Footnote-2::)

   A "macro" can be thought of as an abbreviation you can define for a
collection of control and text lines.  When the macro is "called" by
giving its name after a control character, it is replaced with what it
stands for.  The process of textual replacement is known as
"interpolation".(3)  (*note Requests and Macros-Footnote-3::)
Interpolations are handled as soon as they are recognized, and once
performed, a 'roff' formatter scans the replacement for further
requests, macro calls, and escape sequences.

   In 'roff' systems, the 'de' request defines a macro.(4)  (*note
Requests and Macros-Footnote-4::)

     .de DATE
     2020-11-14
     ..

The foregoing input produces no output by itself; all we have done is
store some information.  Observe the pair of dots that ends the macro
definition.  This is a default; you can specify your own terminator for
the macro definition as the second argument to the 'de' request.

     .de NAME ENDNAME
     Heywood Jabuzzoff
     .ENDNAME

   In fact, the ending marker is itself the name of a macro to be
called, or a request to be invoked, if it is defined at the time its
contol line is read.

     .de END
     Big Rip
     ..
     .de START END
     Big Bang
     .END
     .START
         => Big Rip Big Bang

In the foregoing example, "Big Rip" printed before "Big Bang" because
its macro was _called_ first.  Consider what would happen if we dropped
'END' from the '.de START' line and added '..' after '.END'.  Would the
order change?

   Let us consider a more elaborate example.

     .de DATE
     2020-10-05
     ..
     .
     .de BOSS
     D.\& Kruger,
     J.\& Peterman
     ..
     .
     .de NOTICE
     Approved:
     .DATE
     by
     .BOSS
     ..
     .
     Insert tedious regulatory compliance paragraph here.

     .NOTICE

     Insert tedious liability disclaimer paragraph here.

     .NOTICE
         => Insert tedious regulatory compliance paragraph here.
         =>
         => Approved: 2020-10-05 by D. Kruger, J. Peterman
         =>
         => Insert tedious liability disclaimer paragraph here.
         =>
         => Approved: 2020-10-05 by D. Kruger, J. Peterman

The above document started with a series of control lines.  Three macros
were defined, with a 'de' request declaring each macro's name, and the
"body" of the macro starting on the next line and continuing until a
line with two dots ''..'' marked its end.  The text proper began only
after the macros were defined; this is a common pattern.  Only the
'NOTICE' macro was called "directly" by the document; 'DATE' and 'BOSS'
were called only by 'NOTICE' itself.  Escape sequences were used in
'BOSS', two levels of macro interpolation deep.

   The advantage in typing and maintenance economy may not be obvious
from such a short example, but imagine a much longer document with
dozens of such paragraphs, each requiring a notice of managerial
approval.  Consider what must happen if you are in charge of generating
a new version of such a document with a different date, for a different
boss.  With well-chosen macros, you only have to change each datum in
one place.

   In practice, we would probably use strings (*note Strings::) instead
of macros for such simple interpolations; what is important here is to
glimpse the potential of macros and the power of recursive
interpolation.

   We could have defined 'DATE' and 'BOSS' in the opposite order;
perhaps less obviously, we could also have defined them _after_
'NOTICE'.  "Forward references" like this are acceptable because the
body of a macro definition is not (completely) interpreted, but stored
instead (*note Copy Mode::).  While a macro is being defined (or
appended to), requests are not interpreted and macros not interpolated,
whereas some commonly used escape sequences _are_ interpreted.  'roff'
systems also support recursive macro calls, as long as you have a way to
break the recursion (*note Conditionals and Loops::).  Maintainable
'roff' documents tend to arrange macro definitions to minimize forward
references.

   (1) The <\RET> escape sequence can alter how an input line is
classified; see *note Line Continuation::.

   (2) Argument handling in macros is more flexible but also more
complex.  *Note Calling Macros::.

   (3) Some escape sequences undergo interpolation as well.

   (4) GNU 'troff' offers additional ones.  *Note Writing Macros::.

5.1.8 Macro Packages
--------------------

Macro definitions can be collected into "macro files", 'roff' input
files designed to produce no output themselves but instead ease the
preparation of other 'roff' documents.  There is no syntactical
difference between a macro file and any other 'roff' document; only its
purpose distinguishes it.  When a macro file is installed at a standard
location and suitable for use by a general audience, it is often termed
a "macro package".(1)  (*note Macro Packages-Footnote-1::) Macro
packages can be loaded by supplying the '-m' option to GNU 'troff' or a
'groff' front end.  A document wishing to use a macro package can load
it with the 'mso' ("macro source") request.

   (1) Macro files and packages frequently define registers and strings
as well.

5.1.9 Input Encodings
---------------------

The 'groff' command's '-k' option calls the 'preconv' preprocessor to
perform input character encoding conversions.  Input to the GNU 'troff'
formatter itself, on the other hand, must be in one of two encodings it
can recognize.

'cp1047'
     The code page 1047 input encoding works only on EBCDIC platforms
     (and conversely, the other input encodings don't work with EBCDIC);
     the file 'cp1047.tmac' is loaded at startup.

'latin1'
     ISO Latin-1, an encoding for Western European languages, is the
     default input encoding on non-EBCDIC platforms; the file
     'latin1.tmac' is loaded at startup.

Any document that is encoded in ISO 646:1991 (a descendant of USAS
X3.4-1968 or "US-ASCII"), or, equivalently, uses only code points from
the "C0 Controls" and "Basic Latin" parts of the Unicode character set
is also a valid ISO Latin-1 document; the standards are interchangeable
in their first 128 code points.(1)  (*note Input Encodings-Footnote-1::)

   Other encodings are supported by means of macro packages.

'latin2'
     To use ISO Latin-2, an encoding for Central and Eastern European
     languages, either use '.mso latin2.tmac' at the very beginning of
     your document or use '-mlatin2' as a command-line argument to
     'groff'.

'latin5'
     To use ISO Latin-5, an encoding for the Turkish language, either
     use '.mso latin5.tmac' at the very beginning of your document or
     use '-mlatin5' as a command-line argument to 'groff'.

'latin9'
     ISO Latin-9 is a successor to Latin-1.  Its main difference from
     Latin-1 is that Latin-9 contains the Euro sign.  To use this
     encoding, either use '.mso latin9.tmac' at the very beginning of
     your document or use '-mlatin9' as a command-line argument to
     'groff'.

   Some characters from an input encoding may not be available with a
particular output driver, or their glyphs may not have representation in
the font used.  For terminal devices, fallbacks are defined, like 'EUR'
for the Euro sign and '(C)' for the copyright sign.  For typesetter
devices, you may need to "mount" fonts that support glyphs required by
the document.  *Note Font Positions::.

   Due to the importance of the Euro glyph in Europe, 'groff' is
distributed with a PostScript font called 'freeeuro.pfa', which provides
various glyph shapes for the Euro.  Because standard PostScript fonts
contain the other glyphs from Latin-5 and Latin-9 that Latin-1 lacks,
these encodings are supported for the 'ps' and 'pdf' output devices as
'groff' ships, while Latin-2 is not.

   Unicode supports characters from all other input encodings; the
'utf8' output driver for terminals therefore does as well.  The DVI
output driver supports the Latin-2 and Latin-9 encodings if the
command-line option '-mec' is used as well.  (2)  (*note Input
Encodings-Footnote-2::)

   (1) The _semantics_ of certain punctuation code points have gotten
stricter with the successive standards, a cause of some frustration
among man page writers; see the 'groff_char(7)' man page.

   (2) The DVI output device defaults to using the Computer Modern (CM)
fonts; 'ec.tmac' loads the EC fonts instead, which provide Euro '\[Eu]'
and per mille '\[%0]' glyphs.

5.1.10 Input Conventions
------------------------

Since GNU 'troff' fills text automatically, it is common practice in the
'roff' language to avoid visual composition of text in input files: the
esthetic appeal of the formatted output is what matters.  Therefore,
'roff' input should be arranged such that it is easy for authors and
maintainers to compose and develop the document, understand the syntax
of 'roff' requests, macro calls, and preprocessor languages used, and
predict the behavior of the formatter.  Several traditions have accrued
in service of these goals.

   * Follow sentence endings in the input with newlines to ease their
     recognition (*note Sentences::).  It is frequently convenient to
     end text lines after colons and semicolons as well, as these
     typically precede independent clauses.  Consider doing so after
     commas; they often occur in lists that become easy to scan when
     itemized by line, or constitute supplements to the sentence that
     are added, deleted, or updated to clarify it.  Parenthetical and
     quoted phrases are also good candidates for placement on text lines
     by themselves.

   * Set your text editor's line length to 72 characters or fewer.(1)
     (*note Input Conventions-Footnote-1::) This limit, combined with
     the previous item of advice, makes it less common that an input
     line will wrap in your text editor, and thus will help you perceive
     excessively long constructions in your text.  Recall that natural
     languages originate in speech, not writing, and that punctuation is
     correlated with pauses for breathing and changes in prosody.

   * Use '\&' after '!', '?', and '.' if they are followed by space,
     tab, or newline characters and don't end a sentence.

   * In filled text lines, use '\&' before '.' and ''' if they are
     preceded by space, so that reflowing the input doesn't turn them
     into control lines.

   * Do not use spaces to perform indentation or align columns of a
     table.  Leading spaces are reliable when text is not being filled.

   * Comment your document.  It is never too soon to apply comments to
     record information of use to future document maintainers (including
     your future self).  We thus introduce another escape sequence,
     '\"', which causes GNU 'troff' to ignore the remainder of the input
     line.

   * Use the empty request--a control character followed immediately by
     a newline--to visually manage separation of material in input
     files.  Many of the 'groff' project's own documents use an empty
     request between sentences, after macro definitions, and where a
     break is expected, and two empty requests between paragraphs or
     other requests or macro calls that will introduce vertical space
     into the document.

     You can combine the empty request with the comment escape sequence
     to include whole-line comments in your document, and even "comment
     out" sections of it.

   We conclude this section with an example sufficiently long to
illustrate most of the above suggestions in practice.  For the purpose
of fitting the example between the margins of this manual with the font
used for its typeset version, we have shortened the input line length to
56 columns.  As before, an arrow -> indicates a tab character.

     .\"   nroff this_file.roff | less
     .\"   groff -T ps this_file.roff > this_file.ps
     ->The theory of relativity is intimately connected with
     the theory of space and time.
     .
     I shall therefore begin with a brief investigation of
     the origin of our ideas of space and time,
     although in doing so I know that I introduce a
     controversial subject.  \" remainder of paragraph elided
     .
     .

     ->The experiences of an individual appear to us arranged
     in a series of events;
     in this series the single events which we remember
     appear to be ordered according to the criterion of
     \[lq]earlier\[rq] and \[lq]later\[rq], \" punct swapped
     which cannot be analysed further.
     .
     There exists,
     therefore,
     for the individual,
     an I-time,
     or subjective time.
     .
     This itself is not measurable.
     .
     I can,
     indeed,
     associate numbers with the events,
     in such a way that the greater number is associated with
     the later event than with an earlier one;
     but the nature of this association may be quite
     arbitrary.
     .
     This association I can define by means of a clock by
     comparing the order of events furnished by the clock
     with the order of a given series of events.
     .
     We understand by a clock something which provides a
     series of events which can be counted,
     and which has other properties of which we shall speak
     later.
     .\" Albert Einstein, _The Meaning of Relativity_, 1922

   (1) Emacs: 'fill-column: 72'; Vim: 'textwidth=72'

5.2 Page Geometry
=================

'roff' systems format text under certain assumptions about the size of
the output medium, or page.  For the formatter to correctly break a line
it is filling, it must know the line length, which it derives from the
page width (*note Line Layout::).  For it to decide whether to write an
output line to the current page or wait until the next one, it must know
the page length (*note Page Layout::).

   A device's "resolution" converts practical units like inches or
centimeters to "basic units", a convenient length measure for the output
device or file format.  The formatter and output driver use basic units
to reckon page measurements.  The device description file defines its
resolution and page dimensions (*note DESC File Format::).

   A "page" is a two-dimensional structure upon which a 'roff' system
imposes a rectangular coordinate system with its upper left corner as
the origin.  Coordinate values are in basic units and increase down and
to the right.  Useful ones are therefore always positive and within
numeric ranges corresponding to the page boundaries.

   While the formatter (and, later, output driver) is processing a page,
it keeps track of its "drawing position", which is the location at which
the next glyph will be written, from which the next motion will be
measured, or where a geometric primitive will commence rendering.
Notionally, glyphs are drawn from the text baseline upward and to the
right.(1)  (*note Page Geometry-Footnote-1::) The "text baseline" is a
(usually invisible) line upon which the glyphs of a typeface are
aligned.  A glyph therefore "starts" at its bottom-left corner.  If
drawn at the origin, a typical letter glyph would lie partially or
wholly off the page, depending on whether, like "g", it features a
decender below the baseline.

   Such a situation is nearly always undesirable.  It is furthermore
conventional not to write or draw at the extreme edges of the page.
Therefore the initial drawing position of a 'roff' formatter is not at
the origin, but below and to the right of it.  This rightward shift from
the left edge is known as the "page offset".(2)  (*note Page
Geometry-Footnote-2::) The downward shift leaves room for a text output
line.

   Text is arranged on a one-dimensional lattice of text baselines from
the top to the bottom of the page.  "Vertical spacing" is the distance
between adjacent text baselines.  Typographic tradition sets this
quantity to 120% of the type size.  The initial drawing position is one
unit of vertical spacing below the page top.  Typographers term this
unit a vee.

   Vertical spacing has an impact on page-breaking decisions.
Generally, when a break occurs, the formatter moves the drawing position
to the next text baseline automatically.  If the formatter were already
writing to the last line that would fit on the page, advancing by one
vee would place the next text baseline off the page.  Rather than let
that happen, 'roff' formatters instruct the output driver to eject the
page, start a new one, and again set the drawing position to one vee
below the page top; this is a "page break".

   When the last line of input text corresponds to the last output line
that fits on the page, the break caused by the end of input will also
break the page, producing a useless blank one.  Macro packages keep
users from having to confront this difficulty by setting "traps" (*note
Traps::); moreover, all but the simplest page layouts tend to have
headers and footers, or at least bear vertical margins larger than one
vee.

   (1) 'groff' does not yet support right-to-left scripts.

   (2) 'groff''s terminal output devices have page offsets of zero.

5.3 Measurements
================

The formatter sometimes requires the input of numeric parameters to
specify measurements.  These are specified as integers or decimal
fractions with an optional "scaling unit" suffixed.  A scaling unit is a
letter that immediately follows the last digit of a number.  Digits
after the decimal point are optional.  Measurement expressions include
'10.5p', '11i', and '3.c'.

   Measurements are scaled by the scaling unit and stored internally
(with any fractional part discarded) in basic units.  The device
resolution can therefore be obtained by storing a value of '1i' to a
register.  The only constraint on the basic unit is that it is at least
as small as any other unit.

'u'
     Basic unit.

'i'
     Inch; defined as 2.54 centimeters.

'c'
     Centimeter; a centimeter is about 0.3937 inches.

'p'
     Point; a typesetter's unit used for measuring type size.  There are
     72 points to an inch.

'P'
     Pica; another typesetter's unit.  There are 6 picas to an inch and
     12 points to a pica.

's'
'z'
     *Note Using Fractional Type Sizes::, for a discussion of these
     units.

'f'
     GNU 'troff' defines this unit to scale decimal fractions in the
     interval [0, 1] to 16-bit unsigned integers.  It multiplies a
     quantity by 65,536.  *Note Colors::, for usage.

   The magnitudes of other scaling units depend on the text formatting
parameters in effect.  These are useful when specifying measurements
that need to scale with the typeface or vertical spacing.

'm'
     Em; an em is equal to the current type size in points.  It is named
     thus because it is approximately the width of the letter 'M'.

'n'
     En; an en is one-half em.

'v'
     Vee; recall *note Page Geometry::.

'M'
     Hundredth of an em.

5.3.1 Motion Quanta
-------------------

An output device's basic unit 'u' is not necessarily its smallest
addressable length; 'u' can be smaller to avoid problems with integer
roundoff.  The minimum distances that a device can work with in the
horizontal and vertical directions are termed its "motion quanta".
Measurements are rounded to applicable motion quanta.  Half-quantum
fractions round toward zero.

 -- Register: \n[.H]
 -- Register: \n[.V]
     These read-only registers interpolate the horizontal and vertical
     motion quanta, respectively, of the output device in basic units.

   For example, we might draw short baseline rules on a terminal device
as follows.  *Note Drawing Requests::.

     .tm \n[.H]
         error-> 24
     .nf
     \l'36u' 36u
     \l'37u' 37u
         => _ 36u
         => __ 37u

5.3.2 Default Units
-------------------

A general-purpose register (one created or updated with the 'nr'
request; see *note Registers::) is implicitly dimensionless, or reckoned
in basic units if interpreted in a measurement context.  But it is
convenient for many requests and escape sequences to infer a scaling
unit for an argument if none is specified.  An explicit scaling unit
(not after a closing parenthesis) can override an undesirable default.
Effectively, the default unit is suffixed to the expression if a scaling
unit is not already present.  GNU 'troff''s use of integer arithmetic
should also be kept in mind (*note Numeric Expressions::).

   The 'll' request interprets its argument in ems by default.  Consider
several attempts to set a line length of 3.5 inches when the type size
is 10 points on a terminal device with a resolution of 240 basic units
and horizontal motion quantum of 24.  Some expressions become zero; the
request clamps them to that quantum.

     .ll 3.5i      \" 3.5i (= 840u)
     .ll 7/2       \" 7u/2u -> 3u -> 3m -> 0, clamped to 24u
     .ll (7 / 2)u  \" 7u/2u -> as above
     .ll 7/2i      \" 7u/2i -> 7u/480u -> 0 -> as above
     .ll 7i/2      \" 7i/2u -> 1680u/2m -> 1680u/24u -> 35u
     .ll 7i/2u     \" 3.5i (= 840u)

The safest way to specify measurements is to attach a scaling unit.  To
multiply or divide by a dimensionless quantity, use 'u' as its scaling
unit.

5.4 Numeric Expressions
=======================

A "numeric expression" evaluates to an integer: it can be as simple as a
literal '0' or it can be a complex sequence of register and string
interpolations interleaved with measurements and operators.

   GNU 'troff' provides a set of mathematical and logical operators
familiar to programmers--as well as some unusual ones--but supports only
integer arithmetic.(1)  (*note Numeric Expressions-Footnote-1::) The
internal data type used for computing results is usually a 32-bit signed
integer, which suffices to represent magnitudes within a range of 2
billion.(2)  (*note Numeric Expressions-Footnote-2::)

   Arithmetic infix operators perform a function on the numeric
expressions to their left and right; they are '+' (addition), '-'
(subtraction), '*' (multiplication), '/' (truncating division), and '%'
(modulus).  "Truncating division" rounds to the integer nearer to zero,
no matter how large the fractional portion.  Overflow and division (or
modulus) by zero are errors and abort evaluation of a numeric
expression.

   Arithmetic unary operators operate on the numeric expression to their
right; they are '-' (negation) and '+' (assertion--for completeness; it
does nothing).  The unary minus must often be used with parentheses to
avoid confusion with the decrementation operator, discussed below.

   Observe the rounding behavior and effect of negative operands on the
modulus and truncating division operators.

     .nr T 199/100
     .nr U 5/2
     .nr V (-5)/2
     .nr W 5/-2
     .nr X 5%2
     .nr Y (-5)%2
     .nr Z 5%-2
     T=\n[T] U=\n[U] V=\n[V] W=\n[W] X=\n[X] Y=\n[Y] Z=\n[Z]
         => T=1 U=2 V=-2 W=-2 X=1 Y=-1 Z=1

The sign of the modulus of operands of mixed signs is determined by the
sign of the first.  Division and modulus operators satisfy the following
property: given a dividend A and a divisor B, a quotient Q formed by '(a
/ b)' and a remainder R by '(a % b)', then qb + r = a.

   GNU 'troff''s scaling operator, used with parentheses as '(C;E)',
evaluates a numeric expression E using C as the default scaling unit.
If C is omitted, scaling units are ignored in the evaluation of E.  This
operator can save typing by avoiding the attachment of scaling units to
every operand out of caution.  Your macros can select a sensible default
unit in case the user neglects to supply one.

     .\" Indent by amount given in first argument; assume ens.
     .de Indent
     .  in (n;\\$1)
     ..

Without the scaling operator, the foregoing macro would, if called with
a unitless argument, cause indentation by the 'in' request's default
scaling unit (ems).  The result would be twice as much indentation as
expected.

   GNU 'troff' also provides a pair of operators to compute the extrema
of two operands: '>?' (maximum) and '<?' (minimum).

     .nr slots 5
     .nr candidates 3
     .nr salaries (\n[slots] <? \n[candidates])
     Looks like we'll end up paying \n[salaries] salaries.
         => Looks like we'll end up paying 3 salaries.

   Comparison operators comprise '<' (less than), '>' (greater than),
'<=' (less than or equal), '>=' (greater than or equal), and '='
(equal).  '==' is a synonym for '='.  When evaluated, a comparison is
replaced with '0' if it is false and '1' if true.  In the 'roff'
language, positive values are true, others false.

   We can operate on truth values with the logical operators '&'
(logical conjunction or "and") and ':' (logical disjunction or "or").
They evaluate as comparison operators do.

   A logical complementation ("not") operator, '!', works only within
'if', 'ie', and 'while' requests.  Furthermore, '!' is recognized only
at the beginning of a numeric expression not contained by another
numeric expression.  In other words, it must be the "outermost"
operator.  Including it elsewhere in the expression produces a warning
in the 'number' category (*note Warnings::), and its expression
evaluates false.  This unfortunate limitation maintains compatibility
with AT&T 'troff'.  You can test a numeric expression for falsity by
comparing it to a false value.

     .nr X 1
     .nr Y 0
     .\" This does not work as expected.
     .if (\n[X])&(!\n[Y]) .nop A: X is true, Y is false
     .
     .\" Use this construct instead.
     .if (\n[X])&(\n[Y]<=0) .nop B: X is true, Y is false
         error-> warning: expected numeric expression, got '!'
         => B: X is true, Y is false

   The 'roff' language has no operator precedence: expressions are
evaluated strictly from left to right, in contrast to schoolhouse
arithmetic.  Use parentheses '(' ')' to impose a desired precedence upon
subexpressions.

     .nr X 3+5*4
     .nr Y (3+5)*4
     .nr Z 3+(5*4)
     X=\n[X] Y=\n[Y] Z=\n[Z]
         => X=32 Y=32 Z=23

   For many requests and escape sequences that cause motion on the page,
the unary operators '+' and '-' work differently when leading a numeric
expression.  They then indicate a motion relative to the drawing
position: positive is down in vertical contexts, right in horizontal
ones.

   '+' and '-' are also treated differently by the following requests
and escape sequences: 'bp', 'in', 'll', 'lt', 'nm', 'nr', 'pl', 'pn',
'po', 'ps', 'pvs', 'rt', 'ti', '\H', '\R', and '\s'.  Here, leading plus
and minus signs serve as incrementation and decrementation operators,
respectively.  To negate an expression, subtract it from zero or include
the unary minus in parentheses with its argument.  *Note Setting
Registers::, for examples.

   A leading '|' operator indicates a motion relative not to the drawing
position but to a boundary.  For horizontal motions, the measurement
specifies a distance relative to a drawing position corresponding to the
beginning of the _input_ line.  By default, tab stops reckon movements
in this way.  Most escape sequences do not; '|' tells them to do so.

     Mind the \h'1.2i'gap.
     .br
     Mind the \h'|1.2i'gap.
     .br
     Mind the
     \h'|1.2i'gap.
         => Mind the             gap.
         => Mind the    gap.
         => Mind the             gap.

   One use of this feature is to define macros whose scope is limited to
the output they format.

     .\" underline word $1 with trailing punctuation $2
     .de Underline
     .  nop \\$1\l'|0\[ul]'\\$2
     ..
     Typographical emphasis is best used
     .Underline sparingly .

In the above example, '|0' specifies a negative motion from the current
position (at the end of the argument just emitted, '\$1') to the
beginning of the input line.  Thus, the '\l' escape sequence in this
case draws a line from right to left.  A macro call occurs at the
beginning of an input line;(3) (*note Numeric Expressions-Footnote-3::)
if the '|' operator were omitted, then the underline would be drawn at
zero distance from the current position, producing device-dependent, and
likely undesirable, results.  On the 'ps' output device, it underlines
the period.

   For vertical movements, the '|' operator specifies a distance from
the first text baseline on the page or in the current diversion, using
the current vertical spacing.

     A
     .br
     B \Z'C'\v'|0'D
         => A D
         => B C

   In the foregoing example, we've used the '\Z' escape sequence (*note
Page Motions::) to restore the drawing position after formatting 'C',
then moved vertically to the first text baseline on the page.

 -- Escape sequence: \B'anything'
     Interpolate 1 if ANYTHING is a valid numeric expression, and 0
     otherwise.  The delimiter need not be a neutral apostrophe; see
     *note Delimiters::.

   You might use '\B' along with the 'if' request to filter out invalid
macro or string arguments.  *Note Conditionals and Loops::.

     .\" Indent by amount given in first argument; assume ens.
     .de Indent
     .  if \B'\\$1' .in (n;\\$1)
     ..

   A register interpolated as an operand in a numeric expression must
have an Arabic format; luckily, this is the default.  *Note Assigning
Register Formats::.

   Because spaces separate arguments to requests, spaces are not allowed
in numeric expressions unless the (sub)expression containing them is
surrounded by parentheses.  *Note Invoking Requests::, and *note
Conditionals and Loops::.

     .nf
     .nr a 1+2 + 2+1
     \na
         error-> expected numeric expression, got a space
         => 3
     .nr a 1+(2 + 2)+1
     \na
         => 6

   The 'nr' request (*note Setting Registers::) expects its second and
optional third arguments to be numeric expressions; a bare '+' does not
qualify, so our first attempt got a warning.

   (1) Provision is made for interpreting and reporting decimal
fractions in certain cases.

   (2) If that's not enough, see the 'groff_tmac(5)' man page for the
'62bit.tmac' macro package.

   (3) Control structure syntax creates an exception to this rule, but
is designed to remain useful: recalling our example, '.if 1 .Underline
this' would underline only "this", precisely.  *Note Conditionals and
Loops::.

5.5 Identifiers
===============

GNU 'troff' has rules for properly formed "identifiers"--labels for
objects with syntactical importance, like registers, names (macros,
strings, or diversions), typefaces, glyphs, colors, character classes,
environments, and streams.  An identifier consists of one or more
characters excepting spaces, tabs, newlines, and invalid input
characters.

   Invalid input characters are a subset of control characters (from the
sets "C0 Controls" and "C1 Controls" as Unicode describes them).  When
GNU 'troff' encounters one in an identifier, it produces a warning in
category 'input' (*note Warnings::).  They are removed during
interpretation: an identifier 'foo', followed by an invalid character
and then 'bar', is processed as 'foobar'.

   On a machine using the ISO 646, 8859, or 10646 character encodings,
invalid input characters are '0x00', '0x08', '0x0B', '0x0D'-'0x1F', and
'0x80'-'0x9F'.  On an EBCDIC host, they are '0x00'-'0x01', '0x08',
'0x09', '0x0B', '0x0D'-'0x14', '0x17'-'0x1F', and '0x30'-'0x3F'.(1)
(*note Identifiers-Footnote-1::) Some of these code points are used by
GNU 'troff' internally, making it non-trivial to extend the program to
accept UTF-8 or other encodings that use characters from these
ranges.(2)  (*note Identifiers-Footnote-2::)

   The identifiers 'br', 'PP', 'end-list', 'ref*normal-print', '|',
'@_', and '!"#$%'()*+,-./' are all valid.  Discretion should be
exercised to prevent confusion.  Some care is required with identifiers
starting with '(' or '['.

     .nr x 9
     .nr y 1
     .nr (x 2
     .nr [y 3
     .nr sum1 (\n(x + \n[y])
         error-> a space character is not allowed in an escape
         error->   sequence parameter
     A:2+3=\n[sum1]
     .nr sum2 (\n((x + \n[[y])
     B:2+3=\n[sum2]
     .nr sum3 (\n[(x] + \n([y)
     C:2+3=\n[sum3]
         => A:2+3=1 B:2+3=5 C:2+3=5

An identifier with a closing bracket (']') in its name can't be accessed
with bracket-form escape sequences that expect an identifier as a
parameter.  For example, '\[foo]]' accesses the glyph 'foo', followed by
']' in whatever the surrounding context is, whereas '\C'foo]'' formats a
glyph named 'foo]'.  Similarly, the identifier '(' can't be interpolated
_except_ with bracket forms.

   If you begin a macro, string, or diversion name with either of the
characters '[' or ']', you foreclose use of the 'grefer' preprocessor,
which recognizes '.[' and '.]' as bibliographic reference delimiters.

 -- Escape sequence: \A'anything'
     Interpolate 1 if ANYTHING is a valid identifier, and 0 otherwise.
     The delimiter need not be a neutral apostrophe; see *note
     Delimiters::.  Because invalid input characters are removed (see
     above), invalid identifiers are empty or contain spaces, tabs, or
     newlines.

     You can employ '\A' to validate a macro argument before using it to
     construct another escape sequence or identifier.

          .\" usage: .init-coordinate-pair name val1 val2
          .\" Create a coordinate pair where name!x=val1 and
          .\" name!y=val2.
          .de init-coordinate-pair
          .  if \A'\\$1' \{\
          .    if \B'\\$2' .nr \\$1!x \\$2
          .    if \B'\\$3' .nr \\$1!y \\$3
          .  \}
          ..
          .init-coordinate-pair center 5 10
          The center is at (\n[center!x], \n[center!y]).
          .init-coordinate-pair "poi->nt" trash garbage \" ignored
          .init-coordinate-pair point trash garbage \" ignored
              => The center is at (5, 10).

     In this example, we also validated the numeric arguments; the
     registers 'point!x' and 'point!y' remain undefined.  *Note Numeric
     Expressions:: for the '\B' escape sequence.

   How GNU 'troff' handles the interpretation of an undefined identifier
depends on the context.  There is no way to invoke an undefined request;
such syntax is interpreted as a macro call instead.  If the identifier
is being interpreted as a string, macro, or diversion, GNU 'troff' emits
a warning in category 'mac', defines it as empty, and interpolates
nothing.  If the identifier is being interpreted as a register, GNU
'troff' emits a warning in category 'reg', initializes it to zero, and
interpolates that value.  *Note Warnings::, *note Interpolating
Registers::, and *note Strings::.  Attempting to use an undefined
typeface, glyph, color, character class, environment, or stream
generally provokes an error diagnostic.

   Identifiers for requests, macros, strings, and diversions share one
name space; special characters and character classes another.  No other
object types do.

     .de xxx
     .  nop foo
     ..
     .di xxx
     bar
     .br
     .di
     .
     .xxx
         => bar

The foregoing example shows that GNU 'troff' reuses the identifier
'xxx', changing it from a macro to a diversion.  No warning is emitted,
and the previous contents of 'xxx' are lost.

   (1) Historically, control characters like ASCII STX, ETX, and BEL
(Control+B, Control+C, and Control+G) have been observed in 'roff'
documents, particularly in macro packages employing them as delimiters
with the output comparison operator to try to avoid collisions with the
content of arbitrary user-supplied parameters (*note Operators in
Conditionals::).  We discourage this expedient; in GNU 'troff' it is
unnecessary (outside of compatibility mode) because delimited arguments
are parsed at a different input level than the surrounding context.
*Note Implementation Differences::.

   (2) Consider what happens when a C1 control '0x80'-'0x9F' is
necessary as a continuation byte in a UTF-8 sequence.

5.6 Formatter Instructions
==========================

To support documents that require more than filling, automatic line
breaking and hyphenation, adjustment, and supplemental inter-sentence
space, the 'roff' language offers two means of embedding instructions to
the formatter.

   One is a "request", which begins with a control character and takes
up the remainder of the input line.  Requests often perform relatively
large-scale operations such as setting the page length, breaking the
line, or starting a new page.  They also conduct internal operations
like defining macros.

   The other is an "escape sequence", which begins with the escape
character and can be embedded anywhere in the input, even in arguments
to requests and other escape sequences.  Escape sequences interpolate
special characters, strings, or registers, and handle comparatively
minor formatting tasks like sub- and superscripting.

   Some operations, such as font selection and type size alteration, are
available via both requests and escape sequences.

5.6.1 Control Characters
------------------------

The mechanism of using 'roff''s control characters to invoke requests
and call macros was introduced in *note Requests and Macros::.  Control
characters are recognized only at the beginning of an input line, or at
the beginning of the branch of a control structure request; see *note
Conditionals and Loops::.

   A few requests cause a break implicitly; use the no-break control
character to prevent the break.  Break suppression is its sole
behavioral distinction.  Employing the no-break control character to
invoke requests that don't cause breaks is harmless but poor style.
*Note Manipulating Filling and Adjustment::.

   The control character '.' and the no-break control character ''' can
be changed with the 'cc' and 'c2' requests, respectively.

 -- Request: .cc [c]
     Set the control character to C.  With no argument, the default
     control character '.' is restored.  The identity of the control
     character is associated with the environment (*note
     Environments::).

 -- Request: .c2 [c]
     Set the no-break control character to C.  With no argument, the
     default no-break control character ''' is restored.  The identity
     of the no-break control character is associated with the
     environment (*note Environments::).

   When writing a macro, you might wish to know which control character
was used to call it.

 -- Register: \n[.br]
     This read-only register interpolates 1 if the currently executing
     macro was called using the normal control character and 0
     otherwise.  If a macro is interpolated as a string, the '.br'
     register's value is inherited from the context of the string
     interpolation.  *Note Strings::.

     Use this register to reliably intercept requests that imply breaks.

          .als bp*orig bp
          .de bp
          .  ie \\n[.br] .bp*orig
          .  el          'bp*orig
          ..

     Testing the '.br' register outside of a macro definition makes no
     sense.

5.6.2 Invoking Requests
-----------------------

A control character is optionally followed by tabs and/or spaces and
then an identifier naming a request or macro.  The invocation of an
unrecognized request is interpreted as a macro call.  Defining a macro
with the same name as a request replaces the request.  Deleting a
request name with the 'rm' request makes it unavailable.  The 'als'
request can alias requests, permitting them to be wrapped or
non-destructively replaced.  *Note Strings::.

   There is no general limit on argument length or quantity.  Most
requests take one or more arguments, and ignore any they do not expect.
A request may be separated from its arguments by tabs or spaces, but
only spaces can separate an argument from its successor.  Only one
between arguments is necessary; any excess is ignored.  GNU 'troff' does
not allow tabs for argument separation.(1)  (*note Invoking
Requests-Footnote-1::)

   Generally, a space _within_ a request argument is not relevant, not
meaningful, or is supported by bespoke provisions, as with the 'tl'
request's delimiters (*note Page Layout::).  Some requests, like 'ds',
interpret the remainder of the control line as a single argument.  *Note
Strings::.

   Spaces and tabs immediately after a control character are ignored.
Commonly, authors structure the source of documents or macro files with
them.

     .de center
     .  if \\n[.br] \
     .    br
     .  ce \\$@
     ..
     .
     .
     .de right-align
     .->if \\n[.br] \
     .->->br
     .->rj \\$@
     ..

   If you assign an empty blank line trap, you can separate macro
definitions (or any input lines) with blank lines.

     .de do-nothing
     ..
     .blm do-nothing  \" activate blank line trap

     .de center
     .  if \\n[.br] \
     .    br
     .  ce \\$@
     ..


     .de right-align
     .->if \\n[.br] \
     .->->br
     .->rj \\$@
     ..

     .blm             \" deactivate blank line trap

   *Note Blank Line Traps::.

   (1) In compatibility mode, a space is not necessary after a request
or macro name of two characters' length.  Also, Plan 9 'troff' allows
tabs to separate arguments.

5.6.3 Calling Macros
--------------------

If a macro of the desired name does not exist when called, it is
created, assigned an empty definition, and a warning in category 'mac'
is emitted.  Calling an undefined macro _does_ end a macro definition
naming it as its end macro (*note Writing Macros::).

   To embed spaces _within_ a macro argument, enclose the argument in
neutral double quotes '"'.  Horizontal motion escape sequences are
sometimes a better choice for arguments to be formatted as text.

   Consider calls to a hypothetical section heading macro 'uh'.

     .uh The Mouse Problem
     .uh "The Mouse Problem"
     .uh The\~Mouse\~Problem
     .uh The\ Mouse\ Problem

The first line calls 'uh' with three arguments: 'The', 'Mouse', and
'Problem'.  The remainder call the 'uh' macro with one argument, 'The
Mouse Problem'.  The last solution, using escaped spaces, can be found
in documents prepared for AT&T 'troff'.  It can cause surprise when text
is adjusted, because '\SP' inserts a _fixed-width_, non-breaking space.
GNU 'troff''s '\~' escape sequence inserts an adjustable, non-breaking
space.(1)  (*note Calling Macros-Footnote-1::)

   The foregoing raises the question of how to embed neutral double
quotes or backslashes in macro arguments when _those_ characters are
desired as literals.  In GNU 'troff', the special character escape
sequence '\[rs]' produces a backslash and '\[dq]' a neutral double
quote.

   In GNU 'troff''s AT&T compatibility mode, these characters remain
available as '\(rs' and '\(dq', respectively.  AT&T 'troff' did not
consistently define these special characters, but its descendants can be
made to support them.  *Note Device and Font Description Files::.

   If even that is not feasible, options remain.  To obtain a literal
escape character in a macro argument, you can simply type it if you
change or disable the escape character first.  *Note Using Escape
Sequences::.  Otherwise, you must escape the escape character repeatedly
to a context-dependent extent.  *Note Copy Mode::.

   For the (neutral) double quote, you have recourse to an obscure
syntactical feature of AT&T 'troff'.  Because a double quote can begin a
macro argument, the formatter keeps track of whether the current
argument was started thus, and doesn't require a space after the double
quote that ends it.(2)  (*note Calling Macros-Footnote-2::) In the
argument list to a macro, a double quote that _isn't_ preceded by a
space _doesn't_ start a macro argument.  If not preceded by a double
quote that began an argument, this double quote becomes part of the
argument.  Furthermore, within a quoted argument, a pair of adjacent
double quotes becomes a literal double quote.

     .de eq
     .  tm arg1:\\$1 arg2:\\$2 arg3:\\$3
     .  tm arg4:\\$4 arg5:\\$5 arg6:\\$6
     .. \" 4 backslashes on the next line
     .eq a" "b c" "de"f\\\\g" h""i "j""k"
         error-> arg1:a" arg2:b c arg3:de
         error-> arg4:f\g" arg5:h""i arg6:j"k

   Apart from the complexity of the rules, this traditional solution has
the disadvantage that double quotes don't survive repeated argument
expansion in AT&T 'troff' or GNU 'troff''s compatibility mode.  This can
frustrate efforts to pass such arguments intact through multiple macro
calls.

     .cp 1
     .de eq
     .  tm arg1:\\$1 arg2:\\$2 arg3:\\$3
     .  tm arg4:\\$4 arg5:\\$5 arg6:\\$6
     ..
     .de xe
     .  eq \\$1 \\$2 \\$3 \\$4 \\$5 \\$6
     .. \" 8 backslashes on the next line
     .xe a" "b c" "de"f\\\\\\\\g" h""i "j""k"
         error-> arg1:a" arg2:b arg3:c
         error-> arg4:de arg5:f\g" arg6:h""i

   Outside of compatibility mode, GNU 'troff' doesn't exhibit this
problem because it tracks the nesting depth of interpolations.  *Note
Implementation Differences::.

   (1) '\~' is fairly portable; see *note Other Differences::.

   (2) Strictly, you can neglect to close the last quoted macro
argument, relying on the end of the control line to do so.  We consider
this lethargic practice poor style.

5.6.4 Using Escape Sequences
----------------------------

Whereas requests must occur on control lines, escape sequences can occur
intermixed with text and may appear in arguments to requests, macros,
and other escape sequences.  An escape sequence is introduced by the
escape character, a backslash '\' (but see the 'ec' request below).  The
next character selects the escape's function.

   Escape sequences vary in length.  Some take an argument, and of
those, some have different syntactical forms for a one-character,
two-character, or arbitrary-length argument.  Others accept _only_ an
arbitrary-length argument.  In the former scheme, a one-character
argument follows the function character immediately, an opening
parenthesis '(' introduces a two-character argument (no closing
parenthesis is used), and an argument of arbitrary length is enclosed in
brackets '[]'.  In the latter scheme, the user selects a delimiter
character.  A few escape sequences are idiosyncratic, and support both
of the foregoing conventions ('\s'), designate their own termination
sequence ('\?'), consume input until the next newline ('\!', '\"',
'\#'), or support an additional modifier character ('\s' again, and
'\n').  As with requests, use of some escape sequences in source
documents may interact poorly with a macro package you use; consult its
documentation to learn of "safe" sequences or alternative facilities it
provides to achieve the desired result.

   If an escape character is followed by a character that does not
identify a defined operation, the escape character is ignored (producing
a diagnostic of the 'escape' warning category, which is not enabled by
default) and the following character is processed normally.

     $ groff -Tps -ww
     .nr N 12
     .ds co white
     .ds animal elephant
     I have \fI\nN \*(co \*[animal]s,\f[]
     said \P.\&\~Pseudo Pachyderm.
         error-> warning: escape character ignored before 'P'
         => I have 12 white elephants, said P. Pseudo Pachyderm.

   Escape sequence interpolation is of higher precedence than escape
sequence argument interpretation.  This rule affords flexibility in
using escape sequences to construct parameters to other escape
sequences.

     .ds family C\" Courier
     .ds style I\" oblique
     Choice a typeface \f(\*[family]\*[style]wisely.
         => Choose a typeface wisely.

In the above, the syntax form '\f(' accepts only two characters for an
argument; the example works because the subsequent escape sequences are
interpolated before the selection escape sequence argument is processed,
and strings 'family' and 'style' interpolate one character each.(1)
(*note Using Escape Sequences-Footnote-1::)

   The escape character is nearly always interpreted when encountered;
it is therefore desirable to have a way to interpolate it.

 -- Escape sequence: \e
     Interpolate the escape character.

   To format a backslash glyph on the output, use the '\[rs]' special
character escape sequence.  In macro and string definitions, two further
input sequences '\\' and '\E' come into play, permitting deferred
interpretation of escape sequences.  *Note Copy Mode::.

   Outside of copy mode, escape sequence interpretation can be switched
off and back on.  This procedure can obviate the need to double the
escape character inside macro definitions.  *Note Writing Macros::.
(This approach is not available if your macro needs to interpolate
values at the time it is _defined_--but many do not.)

 -- Request: .eo
     Disable the escape mechanism except in copy mode.  Once this
     request is invoked, no input character is recognized as starting an
     escape sequence in interpretation mode.

 -- Request: .ec [c]
     Recognize C as the escape character.  If C is absent or invalid,
     the default escape character '\' is selected.

     Changing the escape character globally likely breaks macro
     packages, since GNU 'troff' has no mechanism to "intern" macros,
     that is, to convert a macro definition into an internal form
     independent of its representation.(2)  (*note Using Escape
     Sequences-Footnote-2::) When a macro is called, its contents are
     interpreted literally.

     .\" This is a simplified version of the `BR` macro from
     .\" the man(7) macro package.
     .eo
     .de BR
     .  ds result \&
     .  while (\n[.$] >= 2) \{\
     .    as result \fB\$1\fR\$2\"
     .    shift 2
     .  \}
     .  if \n[.$] .as result \fB\$1\"
     \*[result]
     .  rm result
     .  ft R
     ..
     .ec

 -- Request: .ecs
 -- Request: .ecr
     The 'ecs' request stores the escape character for recall with
     'ecr'.  'ecr' sets the escape character to '\' if none has been
     saved.

     Use these requests together to temporarily change the escape
     character.

   (1) The omission of spaces before the comment escape sequences is
necessary; see *note Strings::.

   (2) TeX does have such a mechanism.

5.6.5 Delimiters
----------------

Some escape sequences that require parameters use delimiters.  The
neutral apostrophe ''' is a popular choice and shown in this document.
The neutral double quote '"' is also commonly seen.  Letters, numerals,
and leaders can be used.  Punctuation characters are likely better
choices, except for those defined as infix operators in numeric
expressions; see below.

     \l'1.5i\[bu]' \" draw 1.5 inches of bullet glyphs

   The following escape sequences don't take arguments and thus are
allowed as delimiters: '\SP', '\%', '\|', '\^', '\{', '\}', '\'', '\`',
'\-', '\_', '\!', '\?', '\)', '\/', '\,', '\&', '\:', '\~', '\0', '\a',
'\c', '\d', '\e', '\E', '\p', '\r', '\t', and '\u'.  However, using them
this way is discouraged; they can make the input confusing to read.

   A few escape sequences, '\A', '\b', '\o', '\w', '\X', and '\Z',
accept a newline as a delimiter.  Newlines that serve as delimiters
continue to be recognized as input line terminators.

     A caf\o
     e\(aa
     in Paris
         => A caf in Paris

Use of newlines as delimiters in escape sequences is also discouraged.

   Finally, the escape sequences '\D', '\h', '\H', '\l', '\L', '\N',
'\R', '\s', '\S', '\v', and '\x' prohibit many delimiters.

   * the numerals '0'-'9' and the decimal point '.'

   * the (single-character) operators '+-/*%<>=&:()'

   * the space and tab characters

   * any escape sequences other than '\%', '\:', '\{', '\}', '\'', '\`',
     '\-', '\_', '\!', '\/', '\c', '\e', and '\p'

   Delimiter syntax is complex and flexible primarily for historical
reasons; the foregoing restrictions need be kept in mind mainly when
using 'groff' in AT&T compatibility mode.  GNU 'troff' keeps track of
the nesting depth of escape sequence interpolations, so the only
characters you need to avoid using as delimiters are those that appear
in the arguments you input, not any that result from interpolation.
Typically, ''' works fine.  *Note Implementation Differences::.

     $ groff -Tps
     .de Mw
     .  nr wd \w'\\$1'
     .  tm "\\$1" is \\n(wd units wide.
     ..
     .Mw Wet'suwet'en
     .Mw Wet+200i
     .cp 1 \" turn on compatibility mode
     .Mw Wet'suwet'en
     .Mw Wet'
     .Mw Wet+200i
         error-> "Wet'suwet'en" is 54740 units wide.
         error-> "Wet'+200i" is 42610 units wide.
         error-> "Wet'suwet'en" is 15860 units wide.
         error-> "Wet'" is 15860 units wide.
         error-> "Wet'+200i" is 14415860 units wide.

   We see here that in compatibility mode, the part of the argument
after the ''' delimiter escapes from its context and, if nefariously
crafted, influences the computation of the WD register's value in a
surprising way.

5.7 Comments
============

One of the most common forms of escape sequence is the comment.(1)
(*note Comments-Footnote-1::)

 -- Escape sequence: \"
     Start a comment.  Everything up to the next newline is ignored.

     This may sound simple, but it can be tricky to keep the comments
     from interfering with the appearance of the output.  If the escape
     sequence is to the right of some text or a request, that portion of
     the line is ignored, but spaces preceding it are processed normally
     by GNU 'troff'.  This affects only the 'ds' and 'as' requests and
     their variants.

     One possibly irritating idiosyncrasy is that tabs should not be
     used to vertically align comments in the source document.  Tab
     characters are not treated as separators between a request name and
     its first argument, nor between arguments.

     A comment on a line by itself is treated as a blank line, because
     after eliminating the comment, that is all that remains.

          Test
          \" comment
          Test
              => Test
              =>
              => Test

     To avoid this, it is common to combine the empty request with the
     comment escape sequence as '.\"', causing the input line to be
     ignored.

     Another commenting scheme sometimes seen is three consecutive
     single quotes (''''') at the beginning of a line.  This works, but
     GNU 'troff' emits a warning diagnostic (if enabled) about an
     undefined macro (namely '''').

 -- Escape sequence: \#
     Start a comment; everything up to and including the next newline is
     ignored.  This 'groff' extension was introduced to avoid the
     problems described above.

          Test
          \# comment
          Test
              => Test Test

 -- Request: .ig [end]
     Ignore input until, in the current conditional block (if any),(2)
     (*note Comments-Footnote-2::) the macro END is called at the start
     of a control line, or the control line '..' is encountered if END
     is not specified.  'ig' is parsed as if it were a macro definition,
     but its contents are discarded, not stored.(3)  (*note
     Comments-Footnote-3::)

          hand\c
          .de TX
          fasting
          ..
          .ig TX
          This is part of a large block of input that has been
          temporarily(?) commented out.
          We can restore it simply by removing the .ig request and
          the call of its end macro.
          .TX
              => handfasting

   (1) This claim may be more aspirational than descriptive.

   (2) *Note Conditional Blocks::.

   (3) Exception: auto-incrementing registers defined outside the
ignored region _will_ be modified if interpolated with '\n' inside it.
*Note Auto-increment::.

5.8 Registers
=============

In the 'roff' language, numbers can be stored in "registers".  Many
built-in registers exist, supplying anything from the date to details of
formatting parameters.  You can also define your own.  *Note
Identifiers::, for information on constructing a valid name for a
register.

5.8.1 Setting Registers
-----------------------

Define registers and update their values with the 'nr' request or the
'\R' escape sequence.

 -- Request: .nr ident value
 -- Escape sequence: \R'ident value'
     Set register IDENT to VALUE.  If IDENT doesn't exist, GNU 'troff'
     creates it.  In the '\R' escape sequence, the delimiter need not be
     a neutral apostrophe; see *note Delimiters::.  It also does not
     produce an input token in GNU 'troff'.  *Note Gtroff Internals::.

          .nr a (((17 + (3 * 4))) % 4)
          \n[a]
          .\R'a (((17 + (3 * 4))) % 4)'
          \n[a]
              => 1 1

     (Later, we will discuss additional forms of 'nr' and '\R' that can
     change a register's value after it is dereferenced but before it is
     interpolated.  *Note Auto-increment::.)

     The complete transparency of '\R' can cause surprising effects if
     you use registers like '.k', which get evaluated at the time they
     are accessed.

          .ll 1.6i
          .
          aaa bbb ccc ddd eee fff ggg hhh\R':k \n[.k]'
          .tm :k == \n[:k]
              => :k == 126950
          .
          .br
          .
          aaa bbb ccc ddd eee fff ggg hhh\h'0'\R':k \n[.k]'
          .tm :k == \n[:k]
              => :k == 15000

     If you process this with the PostScript device ('-Tps'), there will
     be a line break eventually after 'ggg' in both input lines.
     However, after processing the space after 'ggg', the partially
     collected line is not overfull yet, so GNU 'troff' continues to
     collect input until it sees the space (or in this case, the
     newline) after 'hhh'.  At this point, the line is longer than the
     line length, and the line gets broken.

     In the first input line, since the '\R' escape sequence leaves no
     traces, the check for the overfull line hasn't been done yet at the
     point where '\R' gets handled, and you get a value for the '.k'
     register that is even greater than the current line length.

     In the second input line, the insertion of '\h'0'' to emit an
     invisible zero-width space forces GNU 'troff' to check the line
     length, which in turn causes the start of a new output line.  Now
     '.k' returns the expected value.

   'nr' and '\R' each have two additional special forms to increment or
decrement a register.

 -- Request: .nr ident +value
 -- Request: .nr ident -value
 -- Escape sequence: \R'ident +value'
 -- Escape sequence: \R'ident -value'
     Increment (decrement) register IDENT by VALUE.  In the '\R' escape
     sequence, the delimiter need not be a neutral apostrophe; see *note
     Delimiters::.

          .nr a 1
          .nr a +1
          \na
              => 2

     A leading minus sign in VALUE is always interpreted as a
     decrementation operator, not an algebraic sign.  To assign a
     register a negative value or the negated value of another register,
     you can force GNU 'troff' to interpret '-' as a negation or minus,
     rather than decrementation, operator: enclose it with its operand
     in parentheses or subtract it from zero.

          .nr a 7
          .nr b 3
          .nr a -\nb
          \na
              => 4
          .nr a (-\nb)
          \na
              => -3
          .nr a 0-\nb
          \na
              => -3

     If a register's prior value does not exist (the register was
     undefined), an increment or decrement is applied as if to 0.

 -- Request: .rr ident
     Remove register IDENT.  If IDENT doesn't exist, the request is
     ignored.  Technically, only the name is removed; the register's
     contents are still accessible under aliases created with 'aln', if
     any.

 -- Request: .rnn ident1 ident2
     Rename register IDENT1 to IDENT2.  If IDENT1 doesn't exist, the
     request is ignored.

 -- Request: .aln new old
     Create an alias NEW for an existing register OLD, causing the names
     to refer to the same stored object.  If OLD is undefined, a warning
     in category 'reg' is produced and the request is ignored.  *Note
     Warnings::, for information about the enablement and suppression of
     warnings.

     To remove a register alias, call 'rr' on its name.  A register's
     contents do not become inaccessible until it has no more names.

5.8.2 Interpolating Registers
-----------------------------

Register contents are interpolated with the '\n' escape sequence.

 -- Escape sequence: \ni
 -- Escape sequence: \n(id
 -- Escape sequence: \n[ident]
     Interpolate register with name IDENT (one-character name I,
     two-character name ID).  '\n' is interpreted even in copy mode
     (*note Copy Mode::).  If the register is undefined, it is created,
     assigned a value of '0', that value is interpolated, and a warning
     in category 'reg' is emitted.  *Note Warnings::, for information
     about the enablement and suppression of warnings.

          .nr a 5
          .nr as \na+\na
          \n(as
              => 10

          .nr a1 5
          .nr ab 6
          .ds str b
          .ds num 1
          \n[a\n[num]]
              => 5
          \n[a\*[str]]
              => 6

5.8.3 Auto-increment
--------------------

Registers can also be incremented or decremented by a configured amount
at the time they are interpolated.  The value of the increment is
specified with a third argument to the 'nr' request, and a special
interpolation syntax is used to alter and then retrieve the register's
value.  Together, these features are called "auto-increment".(1)  (*note
Auto-increment-Footnote-1::)

 -- Request: .nr ident value incr
     Set register IDENT to VALUE and its auto-incrementation amount to
     to INCR.  The '\R' escape sequence doesn't support an INCR
     argument.

   Auto-incrementation is not _completely_ automatic; the '\n' escape
sequence in its basic form never alters the value of a register.  To
apply auto-incrementation to a register, interpolate it with '\n'.

 -- Escape sequence: \n+i
 -- Escape sequence: \n-i
 -- Escape sequence: \n+(id
 -- Escape sequence: \n-(id
 -- Escape sequence: \n+[ident]
 -- Escape sequence: \n-[ident]
     Increment or decrement IDENT (one-character name I, two-character
     name ID) by the register's auto-incrementation value and then
     interpolate the new register value.  If IDENT has no
     auto-incrementation value, interpolate as with '\n'.

     .nr a 0 1
     .nr xx 0 5
     .nr foo 0 -2
     \n+a, \n+a, \n+a, \n+a, \n+a
     .br
     \n-(xx, \n-(xx, \n-(xx, \n-(xx, \n-(xx
     .br
     \n+[foo], \n+[foo], \n+[foo], \n+[foo], \n+[foo]
         => 1, 2, 3, 4, 5
         => -5, -10, -15, -20, -25
         => -2, -4, -6, -8, -10

   To change the increment value without changing the value of a
register, assign the register's value to itself by interpolating it, and
specify the desired increment normally.  Apply an increment of '0' to
disable auto-incrementation of the register.

   (1) A negative auto-increment can be considered an "auto-decrement".

5.8.4 Assigning Register Formats
--------------------------------

A writable register's value can be interpolated in several number
formats.  By default, conventional Arabic numerals are used.  Other
formats see use in sectioning and outlining schemes and alternative page
numbering arrangements.

 -- Request: .af reg fmt
     Use number format FMT when interpolating register REG.  Valid
     number formats are as follows.

     '0...'
          Arabic numerals 0, 1, 2, and so on.  Any decimal digit is
          equivalent to '0'; the formatter merely counts the digits
          specified.  Multiple Arabic numerals in FMT cause
          interpolations to be zero-padded on the left if necessary to
          at least as many digits as specified (interpolations never
          truncate a register value).  A register with format '00'
          interpolates values 1, 2, 3 as '01', '02', '03'.  The default
          format for all writable registers is '0'.

     'I'
          Uppercase Roman numerals: 0, I, II, III, IV, ...

     'i'
          Lowercase Roman numerals: 0, i, ii, iii, iv, ...

     'A'
          Uppercase letters: 0, A, B, C, ..., Z, AA, AB, ...

     'a'
          Lowercase letters: 0, a, b, c, ..., z, aa, ab, ...

     Omitting FMT causes a warning in category 'missing'.  *Note
     Warnings::, for information about the enablement and suppression of
     warnings.  Specifying an unrecognized format is an error.

     Zero values are interpolated as '0' in non-Arabic formats.
     Negative quantities are prefixed with '-' irrespective of format.
     In Arabic formats, the sign supplements the field width.  If REG
     doesn't exist, it is created with a zero value.

          .nr a 10
          .af a 0           \" the default format
          \na,
          .af a I
          \na,
          .af a 321
          .nr a (-\na)
          \na,
          .af a a
          \na
              => 10, X, -010, -j

     The representable extrema in the 'i' and 'I' formats correspond to
     Arabic 39,999.  GNU 'troff' uses 'w' and 'z' to represent 5,000
     and 10,000 in Roman numerals, respectively, following the
     convention of AT&T 'troff'--currently, the correct glyphs for Roman
     numerals five thousand ('U+2181') and ten thousand ('U+2182') are
     not used.

     Assigning the format of a read-only register is an error.  Instead,
     copy the read-only register's value to, and assign the format of, a
     writable register.

 -- Escape sequence: \gr
 -- Escape sequence: \g(rg
 -- Escape sequence: \g[reg]
     Interpolate the format of the register REG (one-character name R,
     two-character name RG).  Zeroes represent Arabic formats.  If REG
     is not defined, REG is not created and nothing is interpolated.
     '\g' is interpreted even in copy mode (*note Copy Mode::).

   GNU 'troff' interprets only Arabic numerals.  The Roman numeral or
alphabetic formats cannot be used as operands to arithmetic operators in
expressions (*note Numeric Expressions::).  For instance, it may be
desirable to test the page number independently of its format.

     .af % i \" front matter
     .de header-trap
     .  \" To test the page number, we need it in Arabic.
     .  ds saved-page-number-format \\g%\"
     .  af % 0
     .  nr page-number-in-decimal \\n%
     .  af % \\*[saved-page-number-format]
     .  ie \\n[page-number-in-decimal]=1 .do-first-page-stuff
     .  el \{\
     .    ie o .do-odd-numbered-page-stuff
     .    el   .do-even-numbered-page-stuff
     .  \}
     .  rm saved-page-number-format
     ..
     .wh 0 header-trap

5.8.5 Built-in Registers
------------------------

Predefined registers whose identifiers start with a dot are read-only.
Many are Boolean-valued, interpolating a true or false value testable
with the 'if', 'ie', or 'while' requests.  Some read-only registers are
string-valued, meaning that they interpolate text.

   A register name (without the dot) is often associated with a request
of the same name.  A complete listing of all built-in registers can be
found in *note Register Index::.

   We present here a few built-in registers that are not described
elsewhere in this manual; they have to do with invariant properties of
GNU 'troff', or obtain information about the formatter's command-line
options or the operating environment.  Date- and time-related registers
are set per the local time as determined by 'localtime(3)' when the
formatter launches.  This initialization can be overridden by
'SOURCE_DATE_EPOCH' and 'TZ'; see *note Environment::.

'\n[.F]'
     Name of input file (string-valued).

'\n[.R]'
     Count of available unused registers; always 10,000 in GNU
     'troff'.(1)  (*note Built-in Registers-Footnote-1::)

'\n[.U]'
     Unsafe mode enablement (Boolean-valued); see 'groff' '-U' option
     (*note Groff Options::).

'\n[seconds]'
     Count of seconds elapsed in the minute (0-60).

'\n[minutes]'
     Count of minutes elapsed in the hour (0-59).

'\n[hours]'
     Count of hours elapsed since midnight (0-23).

'\n[dw]'
     Day of the week (1-7; 1 is Sunday).

'\n[dy]'
     Day of the month (1-31).

'\n[mo]'
     Month of the year (1-12).

'\n[year]'
     Gregorian year.

'\n[yr]'
     Gregorian year minus 1900.  This register is incorrectly documented
     in the AT&T 'troff' manual as storing the last two digits of the
     current year.  That claim stopped being true in 2000.  Old 'troff'
     input that looks like:

          '\" The year number is a surprise after 1999.
          This document was formatted in 19\n(yr.

     can be corrected to:

          This document was formatted in \n[year].

     or, for portability across many 'roff' programs, to the following.

          .nr y4 1900+\n(yr
          This document was formatted in \n(y4.

'\n[.c]'
'\n[c.]'
     Input line number.  'c.' is a writable synonym, affecting
     subsequent interpolations of both '.c' and 'c.'.

'\n[.x]'
     Major version number of the running GNU 'troff' formatter.  For
     example, if the version number is 1.23.0, then '.x' contains '1'.

'\n[.y]'
     Minor version number of the running GNU 'troff' formatter.  For
     example, if the version number is 1.23.0, then '.y' contains '23'.

'\n[.Y]'
     Revision number of the running GNU 'troff' formatter.  For example,
     if the version number is 1.23.0, then '.Y' contains '0'.

'\n[$$]'
     Process identifier (PID) of the GNU 'troff' program in its
     operating environment.

'\n[.g]'
     Always true in GNU 'troff' (Boolean-valued).  Documents can use
     this to ask the formatter if it claims 'groff' compatibility.

'\n[.A]'
     Approximate output is being formatted (Boolean-valued); see 'groff'
     '-a' option (*note Groff Options::).

'\n[.P]'
     Output page selection status (Boolean-valued); see 'groff' '-o'
     option (*note Groff Options::).

'\n[.T]'
     Indicator of output device selection (Boolean-valued); see 'groff'
     '-T' option (*note Groff Options::).

   (1) GNU 'troff' dynamically allocates memory for as many registers as
required.

5.9 Manipulating Filling and Adjustment
=======================================

When an output line is pending (see below), a break moves the drawing
position to the beginning of the next text baseline, interrupting
filling.  Various ways of causing breaks were shown in *note Breaking::.
The 'br' request likewise causes a break.  Several other requests imply
breaks: 'bp', 'ce', 'cf', 'fi', 'fl', 'in', 'nf', 'rj', 'sp', 'ti', and
'trf'.  If the no-break control character is used with any of these
requests, GNU 'troff' suppresses the break; instead the requested
operation takes effect at the next break.  ''br' does nothing.

     .ll 55n
     This line is normally filled and adjusted.
     .br
     A line's alignment is decided
     'ce \" Center the next input line (no break).
     when it is output.
     This line returns to normal filling and adjustment.
         => This line is normally filled and adjusted.
         =>    A line's alignment is decided when it is output.
         => This line returns to normal filling and adjustment.

Output line properties like page offset, indentation, and adjustment are
not determined until the line has been broken.  An output line is said
to be "pending" if some input has been collected but an output line
corresponding to it has not yet been written; such an output line is
also termed "partially collected".  If no output line is pending, it is
as if a break has already happened; additional breaks, whether explicit
or implicit, have no effect.  If the vertical drawing position is
negative--as it is when the formatter starts up--a break starts a new
page (even if no output line is pending) unless an end-of-input macro is
being interpreted.  *Note End-of-input Traps::.

 -- Request: .br
     Break the line: emit any pending output line without adjustment.

          foo bar
          .br
          baz
          'br
          qux
              => foo bar
              => baz qux

   Sometimes you want to prevent a break within a phrase or between a
quantity and its units.

 -- Escape sequence: \~
     Insert an unbreakable space that is adjustable like an ordinary
     space.  It is discarded from the end of an output line if a break
     is forced.

          Set the output speed to\~1.
          There are 1,024\~bytes in 1\~KiB.
          J.\~F.\~Ossanna wrote the original CSTR\~#54.

   By default, GNU 'troff' fills text and adjusts it to both margins.
Filling can be disabled via the 'nf' request and re-enabled with the
'fi' request.

 -- Request: .fi
 -- Register: \n[.u]
     Enable filling of output lines; a pending output line is broken.
     The read-only register '.u' is set to 1.  The filling enablement
     status, sometimes called "fill mode", is associated with the
     environment (*note Environments::).  *Note Line Continuation::, for
     interaction with the '\c' escape sequence.

 -- Request: .nf
     Disable filling of output lines: the output line length (*note Line
     Layout::) is ignored and output lines are broken where the input
     lines are.  A pending output line is broken and adjustment is
     suppressed.  The read-only register '.u' is set to 0.  The filling
     enablement status is associated with the environment (*note
     Environments::).  See *note Line Continuation::, for interaction
     with the '\c' escape sequence.

 -- Request: .ad [mode]
 -- Register: \n[.j]
     Enable output line adjustment in MODE, taking effect when the
     pending (or next) output line is broken.  Adjustment is suppressed
     when filling is.  MODE can have one of the following values.

     'b'
     'n'
          Adjust "normally": to both margins.  This is the GNU 'troff'
          default.

     'c'
          Center filled text.  Contrast with the 'ce' request, which
          centers text without filling it.

     'l'
          Align text to the left margin, producing what is sometimes
          called ragged-right text.

     'r'
          Align text to the right margin, producing ragged-left text.

     MODE can also be a value previously stored in the '.j' register.
     Using 'ad' without an argument is the same as '.ad \n[.j]'; unless
     filling is disabled, GNU 'troff' resumes adjusting lines in the
     same way it did before adjustment was disabled by invocation of the
     'na' request.

          .ll 48n
          .de AD
          .  br
          .  ad \\$1
          ..
          .
          .de NA
          .  br
          .  na
          ..
          .
          left
          .AD r
          .nr ad \n(.j
          right
          .AD c
          center
          .NA
          left
          .AD
          center
          .AD \n(ad
          right
              => left
              =>                                            right
              =>                      center
              => left
              =>                      center
              =>                                            right

     The adjustment mode and enablement status are encoded in the
     read-only register '.j'.  These parameters are associated with the
     environment (*note Environments::).

     The value of '.j' for any adjustment mode is an implementation
     detail and should not be relied upon as a programmer's interface.
     Do not write logic to interpret or perform arithmetic on it.

 -- Request: .na
     Disable output line adjustment.  This produces the same output as
     alignment to the left margin, but the value of the adjustment mode
     register '.j' is altered differently.  The adjustment mode and
     enablement status are associated with the environment (*note
     Environments::).

 -- Request: .brp
 -- Escape sequence: \p
     Break, adjusting the line per the current adjustment mode.  '\p'
     schedules a break with adjustment at the next word boundary.  The
     escape sequence is itself neither a break nor a space of any kind;
     it can thus be placed in the middle of a word to cause a break at
     the end of that word.

     Breaking with immediate adjustment can produce ugly results since
     GNU 'troff' doesn't have a sophisticated paragraph-building
     algorithm, as TeX has, for example.  Instead, GNU 'troff' fills and
     adjusts a paragraph line by line.

          .ll 4.5i
          This is an uninteresting sentence.
          This is an uninteresting sentence.\p
          This is an uninteresting sentence.

     is formatted as follows.

          This  is  an uninteresting sentence.  This is
          an          uninteresting           sentence.
          This is an uninteresting sentence.

   To clearly present the next couple of requests, we must introduce the
concept of "productive" input lines.  A "productive input line" is one
that directly produces formatted output.  Text lines produce output, as
do control lines containing requests like 'tl' or escape sequences like
'\D'.  Macro calls are not _directly_ productive, and thus not counted,
but their interpolated contents can be.  Empty requests, and requests
and escape sequences that define registers or strings or alter the
formatting environment (as with changes to the size, face, height,
slant, or color of the type) are not productive.  The output line
continuation escape sequence '\c' "connects" two input lines that would
otherwise be counted separately.  *Note Line Continuation::.

     .de hello
     Hello, world!
     ..
     .ce \" center output of next productive input line
     .
     .nr junk-reg 1
     .ft I
     Chorus: \c
     .ft
     .hello
     Went the day well?
       =>                  Chorus: Hello, world!
       => Went the day well?

 -- Request: .ce [nnn]
 -- Register: \n[.ce]
     Break (unless the no-break control character is used), center the
     output of the next NNN productive input lines without filling, then
     break again (regardless of the control character).  The count of
     lines remaining to be centered is stored in the read-only register
     '.ce' and is associated with the environment (*note
     Environments::).

     While the '.ad c' request also centers text, it fills the text as
     well.  The following example demonstrates the difference.

          .de FR
          This is a small text fragment that shows the differences
          between the `.ce' and the `.ad c' requests.
          ..
          .ll 4i
          .ce 1000
          .FR
          .ce 0

          .ad c
          .FR
              => This is a small text fragment that shows
              =>              the differences
              => between the `.ce' and the `.ad c' requests.
              =>
              => This is a small text fragment that shows
              =>  the differences between the `.ce' and
              =>         the `.ad c' requests.

     With no arguments, 'ce' centers the next line of text.  NNN
     specifies the number of lines to be centered.  If the argument is
     zero or negative, centering is disabled.

     The basis for centering text is the line length (as set with the
     'll' request) minus the indentation (as set with the 'in' request).
     Temporary indentation is ignored.

     The previous example illustrates a common idiom of turning
     centering on for a quantity of lines far in excess of what is
     required, and off again after the text to be centered.  This
     technique relieves humans of counting lines for requests that take
     a count of input lines as an argument.

 -- Request: .rj [nnn]
 -- Register: \n[.rj]
     Break (unless the no-break control character is used), align the
     output of the next NNN productive input lines to the right margin
     without filling, then break again (regardless of the control
     character).  The count of lines remaining to be centered is stored
     in the read-only register '.rj' and is associated with the
     environment (*note Environments::).

 -- Request: .ss word-space-size [additional-sentence-space-size]
 -- Register: \n[.ss]
 -- Register: \n[.sss]
     Set the sizes of spaces between words and sentences.(1)  (*note
     Manipulating Filling and Adjustment-Footnote-1::) Their units are
     twelfths of the space width of the current font.  Initially both
     the WORD-SPACE-SIZE and ADDITIONAL-SENTENCE-SPACE-SIZE are 12.
     Negative values are not permitted.  The request is ignored if there
     are no arguments.

     The first argument, the inter-word space size, is a minimum; if an
     output line undergoes adjustment, such spaces may increase in
     width.

     The optional second argument sets the amount of additional space
     separating sentences on the same output line.  If omitted, this
     amount is set to WORD-SPACE-SIZE.

     The read-only registers '.ss' and '.sss' hold the values of minimal
     inter-word space and additional inter-sentence space, respectively.
     These parameters are associated with the environment (*note
     Environments::), and rounded down to the nearest multiple of 12 on
     terminal output devices.

     Additional inter-sentence space is used only if the output line is
     not full when the end of a sentence occurs in the input.  If a
     sentence ends at the end of an input line, then both an inter-word
     space and an inter-sentence space are added to the output; if two
     spaces follow the end of a sentence in the middle of an input line,
     then the second space becomes an inter-sentence space in the
     output.  Additional inter-sentence space is not adjusted, but the
     inter-word space that always precedes it may be.  Further input
     spaces after the second, if present, are adjusted as normal.

     A related application of the 'ss' request is to insert discardable
     horizontal space; i.e., space that is discarded at a line break.
     For example, some footnote styles collect the notes into a single
     paragraph with large spaces between each.

          .ll 48n
          1.\~J. Fict. Ch. Soc. 6 (2020), 3\[en]14.
          .ss 12 48 \" applies to next sentence ending
          Reprints no longer available through FCS.
          .ss 12 \" go back to normal
          2.\~Better known for other work.
              => 1.  J.  Fict. Ch. Soc. 6 (2020), 3-14.  Reprints
              => no longer available through FCS.      2.  Better
              => known for other work.

     If _undiscardable_ space is required, use the '\h' escape sequence.

   (1) *Note Filling:: and *note Sentences:: for the definitions of word
and sentence boundaries, respectively.

5.10 Manipulating Hyphenation
=============================

When filling, GNU 'troff' hyphenates words as needed at user-specified
and automatically determined hyphenation points.  The machine-driven
determination of hyphenation points in words requires algorithms and
data, and is susceptible to conventions and preferences.  Before
tackling such "automatic hyphenation", let us consider how hyphenation
points can be set manually.

   Explicitly hyphenated words such as "mother-in-law" are always
eligible for breaking after each of their hyphens.  Relatively few words
in a language offer such obvious break points, however, and automatic
hyphenation is not perfect, particularly for unusual words found in
technical literature.  We may wish to instruct GNU 'troff' how to
hyphenate specific words if the need arises.

 -- Request: .hw word ...
     Define each "hyphenation exception" WORD with each hyphen '-' in
     the word indicating a hyphenation point.  For example, the request

          .hw in-sa-lub-rious alpha

     marks potential hyphenation points in "insalubrious", and prevents
     "alpha" from being hyphenated at all.

     Besides the space character, any character whose hyphenation code
     is zero can be used to separate the arguments of 'hw' (see the
     'hcode' request below).  In addition, this request can be used more
     than once.

     Hyphenation points specified with 'hw' are not subject to the
     within-word placement restrictions imposed by the 'hy' request (see
     below).

     Hyphenation exceptions specified with the 'hw' request are
     associated with the hyphenation language (see the 'hla' request
     below) and environment (*note Environments::); invoking the 'hw'
     request in the absence of a hyphenation language is an error.

     The request is ignored if there are no parameters.

   These are known as hyphenation exceptions in the expectation that
most users will avail themselves of automatic hyphenation; these
exceptions override any rules that would normally apply to a word
matching a hyphenation exception defined with 'hw'.

   Situations also arise when only a specific occurrence of a word needs
its hyphenation altered or suppressed, or when something that is not a
word in a natural language, like a URL, needs to be breakable in
sensible places without hyphens.

 -- Escape sequence: \%
 -- Escape sequence: \:
     To tell GNU 'troff' how to hyphenate words as they occur in input,
     use the '\%' escape sequence; it is the default "hyphenation
     character".  Each instance within a word indicates to GNU 'troff'
     that the word may be hyphenated at that point, while prefixing a
     word with this escape sequence prevents it from being otherwise
     hyphenated.  This mechanism affects only that occurrence of the
     word; to change the hyphenation of a word for the remainder of
     input processing, use the 'hw' request.

     GNU 'troff' regards the escape sequences '\X' and '\Y' as starting
     a word; that is, the '\%' escape sequence in, say,
     '\X'...'\%foobar' or '\Y'...'\%foobar' no longer prevents
     hyphenation of 'foobar' but inserts a hyphenation point just prior
     to it; most likely this isn't what you want.  *Note Postprocessor
     Access::.

     '\:' inserts a non-printing break point; that is, a word can break
     there, but the soft hyphen glyph (see below) is not written to the
     output if it does.  This escape sequence is an input word boundary,
     so the remainder of the word is subject to hyphenation as normal.

     You can use '\:' and '\%' in combination to control breaking of a
     file name or URL or to permit hyphenation only after certain
     explicit hyphens within a word.

          The \%Lethbridge-Stewart-\:\%Sackville-Baggins divorce
          was, in retrospect, inevitable once the contents of
          \%/var/log/\:\%httpd/\:\%access_log on the family web
          server came to light, revealing visitors from Hogwarts.

 -- Request: .hc [char]
     Change the hyphenation character to CHAR.  This character then
     works as the '\%' escape sequence normally does, and thus no longer
     appears in the output.(1)  (*note Manipulating
     Hyphenation-Footnote-1::) Without an argument, 'hc' resets the
     hyphenation character to '\%' (the default).  The hyphenation
     character is associated with the environment (*note
     Environments::).

 -- Request: .shc [glyph]
     Set the "soft hyphen character",(2) (*note Manipulating
     Hyphenation-Footnote-2::) inserted when a word is hyphenated
     automatically or at a hyphenation character, to GLYPH.(3)  (*note
     Manipulating Hyphenation-Footnote-3::) If the argument is omitted,
     the soft hyphen glyph is set to the default, '\[hy]'.  If the
     selected glyph does not exist in the font in use at a potential
     hyphenation point, then the line is not broken at that point.
     Neither character definitions (specified with the 'char' and
     similar requests) nor translations (specified with the 'tr'
     request) are considered when assigning the soft hyphen glyph.

   Several requests influence automatic hyphenation.  Because
conventions vary, a variety of hyphenation modes is available to the
'hy' request; these determine whether hyphenation will apply to a word
prior to breaking a line at the end of a page (more or less; see below
for details), and at which positions within that word automatically
determined hyphenation points are permissible.  The places within a word
that are eligible for hyphenation are determined by language-specific
data and lettercase relationships.  Furthermore, hyphenation of a word
might be suppressed due to a limit on consecutive hyphenated lines
('hlm'), a minimum line length threshold, certain minimum length
('hym'), or because the line can instead be adjusted with additional
inter-word space ('hys').

 -- Request: .hy [mode]
 -- Register: \n[.hy]
     Set automatic hyphenation mode to MODE, an integer encoding
     conditions for hyphenation; if omitted, '1' is implied.  The
     hyphenation mode is available in the read-only register '.hy'; it
     is associated with the environment (*note Environments::).  The
     default hyphenation mode depends on the localization file loaded
     when GNU 'troff' starts up; see the 'hpf' request below.

     Typesetting practice generally does not avail itself of every
     opportunity for hyphenation, but the details differ by language and
     site mandates.  The hyphenation modes of AT&T 'troff' were
     implemented with English-language publishing practices of the 1970s
     in mind, not a scrupulous enumeration of conceivable parameters.
     GNU 'troff' extends those modes such that finer-grained control is
     possible, favoring compatibility with older implementations over a
     more intuitive arrangement.  The means of hyphenation mode control
     is a set of numbers that can be added up to encode the behavior
     sought.(4)  (*note Manipulating Hyphenation-Footnote-4::) The
     entries in the following table are termed "values"; the sum of the
     desired values is the "mode".

     '0'
          disables hyphenation.

     '1'
          enables hyphenation except after the first and before the last
          character of a word.

     The remaining values "imply" 1; that is, they enable hyphenation
     under the same conditions as '.hy 1', and then apply or lift
     restrictions relative to that basis.

     '2'
          disables hyphenation of the last word on a page,(5) (*note
          Manipulating Hyphenation-Footnote-5::) even for manually
          hyphenated words.

     '4'
          disables hyphenation before the last two characters of a word.

     '8'
          disables hyphenation after the first two characters of a word.

     '16'
          enables hyphenation before the last character of a word.

     '32'
          enables hyphenation after the first character of a word.

     Apart from value 2, restrictions imposed by the hyphenation mode
     are _not_ respected for words whose hyphenations have been
     specified with the hyphenation character ('\%' by default) or the
     'hw' request.

     Nonzero values in the previous table are additive.  For example,
     mode 12 causes GNU 'troff' to hyphenate neither the last two nor
     the first two characters of a word.  Some values cannot be used
     together because they contradict; for instance, values 4 and 16,
     and values 8 and 32.  As noted, it is superfluous to add 1 to any
     nonzero even mode.

     The automatic placement of hyphens in words is determined by
     "pattern files", which are derived from TeX and available for
     several languages.  The number of characters at the beginning of a
     word after which the first hyphenation point should be inserted is
     determined by the patterns themselves; it can't be reduced further
     without introducing additional, invalid hyphenation points
     (unfortunately, this information is not part of a pattern file--you
     have to know it in advance).  The same is true for the number of
     characters at the end of a word before the last hyphenation point
     should be inserted.  For example, you can supply the following
     input to 'echo $(nroff)'.

          .ll 1
          .hy 48
          splitting

     You will get

          s- plit- t- in- g

     instead of the correct 'split- ting'.  English patterns as
     distributed with GNU 'troff' need two characters at the beginning
     and three characters at the end; this means that value 4 of 'hy' is
     mandatory.  Value 8 is possible as an additional restriction, but
     values 16 and 32 should be avoided, as should mode 1.  Modes 4
     and 6 are typical.

     A table of left and right minimum character counts for hyphenation
     as needed by the patterns distributed with GNU 'troff' follows; see
     the 'groff_tmac(5)' man page for more information on GNU 'troff''s
     language macro files.

     language             pattern name   left min   right min
     -----------------------------------------------------------
     Czech                cs             2          2
     English              en             2          3
     French               fr             2          3
     German traditional   det            2          2
     German reformed      den            2          2
     Italian              it             2          2
     Swedish              sv             1          2

     Hyphenation exceptions within pattern files (i.e., the words within
     a TeX '\hyphenation' group) obey the hyphenation restrictions given
     by 'hy'.

 -- Request: .nh
     Disable automatic hyphenation; i.e., set the hyphenation mode to 0
     (see above).  The hyphenation mode of the last call to 'hy' is not
     remembered.

 -- Request: .hpf pattern-file
 -- Request: .hpfa pattern-file
 -- Request: .hpfcode a b [c d] ...
     Read hyphenation patterns from PATTERN-FILE, which is sought in the
     same way that macro files are with the 'mso' request or the
     '-mNAME' command-line option to 'groff'.  The PATTERN-FILE should
     have the same format as (simple) TeX pattern files.  More
     specifically, the following scanning rules are implemented.

        * A percent sign starts a comment (up to the end of the line)
          even if preceded by a backslash.

        * "Digraphs" like '\$' are not supported.

        * '^^XX' (where each X is 0-9 or a-f) and '^^C' (character C in
          the code point range 0-127 decimal) are recognized; other uses
          of '^' cause an error.

        * No macro expansion is performed.

        * 'hpf' checks for the expression '\patterns{...}' (possibly
          with whitespace before or after the braces).  Everything
          between the braces is taken as hyphenation patterns.
          Consequently, '{' and '}' are not allowed in patterns.

        * Similarly, '\hyphenation{...}' gives a list of hyphenation
          exceptions.

        * '\endinput' is recognized also.

        * For backward compatibility, if '\patterns' is missing, the
          whole file is treated as a list of hyphenation patterns
          (except that the '%' character is recognized as the start of a
          comment).

     The 'hpfa' request appends a file of patterns to the current list.

     The 'hpfcode' request defines mapping values for character codes in
     pattern files.  It is an older mechanism no longer used by GNU
     'troff''s own macro files; for its successor, see 'hcode' below.
     'hpf' or 'hpfa' apply the mapping after reading the patterns but
     before replacing or appending to the active list of patterns.  Its
     arguments are pairs of character codes--integers from 0 to 255.
     The request maps character code A to code B, code C to code D, and
     so on.  Character codes that would otherwise be invalid in GNU
     'troff' can be used.  By default, every code maps to itself except
     those for letters 'A' to 'Z', which map to those for 'a' to 'z'.

     The set of hyphenation patterns is associated with the language set
     by the 'hla' request (see below).  The 'hpf' request is usually
     invoked by a localization file loaded by the 'troffrc' file.(6)
     (*note Manipulating Hyphenation-Footnote-6::)

     A second call to 'hpf' (for the same language) replaces the
     hyphenation patterns with the new ones.  Invoking 'hpf' or 'hpfa'
     causes an error if there is no hyphenation language.  If no 'hpf'
     request is specified (either in the document, in a file loaded at
     startup, or in a macro package), GNU 'troff' won't automatically
     hyphenate at all.

 -- Request: .hcode c1 code1 [c2 code2] ...
     Set the hyphenation code of character C1 to CODE1, that of C2 to
     CODE2, and so on.  A hyphenation code must be an ordinary character
     (not a special character escape sequence) other than a digit or a
     space.  The request is ignored if given no arguments.

     For hyphenation to work, hyphenation codes must be set up.  At
     startup, GNU 'troff' assigns hyphenation codes to the letters
     'a'-'z' (mapped to themselves), to the letters 'A'-'Z' (mapped to
     'a'-'z'), and zero to all other characters.  Normally, hyphenation
     patterns contain only lowercase letters which should be applied
     regardless of case.  In other words, they assume that the words
     'FOO' and 'Foo' should be hyphenated exactly as 'foo' is.  The
     'hcode' request extends this principle to letters outside the
     Unicode basic Latin alphabet; without it, words containing such
     letters won't be hyphenated properly even if the corresponding
     hyphenation patterns contain them.

     For example, the following 'hcode' requests are necessary to assign
     hyphenation codes to the letters '', needed for German.

          .hcode     
          .hcode     
          .hcode     
          .hcode  

     Without these assignments, GNU 'troff' treats the German word
     'Kindergrten' (the plural form of 'kindergarten') as two words
     'kinderg' and 'rten' because the hyphenation code of the umlaut a
     is zero by default, just like a space.  There is a German
     hyphenation pattern that covers 'kinder', so GNU 'troff' finds the
     hyphenation 'kin-der'.  The other two hyphenation points
     ('kin-der-gr-ten') are missed.

 -- Request: .hla lang
 -- Register: \n[.hla]
     Set the hyphenation language to LANG.  Hyphenation exceptions
     specified with the 'hw' request and hyphenation patterns and
     exceptions specified with the 'hpf' and 'hpfa' requests are
     associated with the hyphenation language.  The 'hla' request is
     usually invoked by a localization file, which is turn loaded by the
     'troffrc' or 'troffrc-end' file; see the 'hpf' request above.

     The hyphenation language is available in the read-only
     string-valued register '.hla'; it is associated with the
     environment (*note Environments::).

 -- Request: .hlm [n]
 -- Register: \n[.hlm]
 -- Register: \n[.hlc]
     Set the maximum quantity of consecutive hyphenated lines to N.  If
     N is negative, there is no maximum.  If omitted, N is -1.  This
     value is associated with the environment (*note Environments::).
     Only lines output from a given environment count toward the maximum
     associated with that environment.  Hyphens resulting from '\%' are
     counted; explicit hyphens are not.

     The '.hlm' read-only register stores this maximum.  The count of
     immediately preceding consecutive hyphenated lines is available in
     the read-only register '.hlc'.

 -- Request: .hym [length]
 -- Register: \n[.hym]
     Set the (right) hyphenation margin to LENGTH.  If the adjustment
     mode is not 'b' or 'n', the line is not hyphenated if it is shorter
     than LENGTH.  Without an argument, the hyphenation margin is reset
     to its default value, 0.  The default scaling indicator is 'm'.
     The hyphenation margin is associated with the environment (*note
     Environments::).

     A negative argument resets the hyphenation margin to zero, emitting
     a warning in category 'range'.

     The hyphenation margin is available in the '.hym' read-only
     register.

 -- Request: .hys [hyphenation-space]
 -- Register: \n[.hys]
     Suppress hyphenation of the line in adjustment modes 'b' or 'n' if
     it can be justified by adding no more than HYPHENATION-SPACE extra
     space to each inter-word space.  Without an argument, the
     hyphenation space adjustment threshold is set to its default value,
     0.  The default scaling indicator is 'm'.  The hyphenation space
     adjustment threshold is associated with the environment (*note
     Environments::).

     A negative argument resets the hyphenation space adjustment
     threshold to zero, emitting a warning in category 'range'.

     The hyphenation space adjustment threshold is available in the
     '.hys' read-only register.

   (1) '\%' itself stops marking hyphenation points but still produces
no output glyph.

   (2) "Soft hyphen _character_" is a misnomer since it is an output
glyph.

   (3) "Soft" because it appears in output only where a hyphenation
break is performed; a "hard" hyphen, as in "long-term", always appears.

   (4) The mode is a vector of Booleans encoded as an integer.  To a
programmer, this fact is easily deduced from the exclusive use of powers
of two for the configuration parameters; they are computationally easy
to "mask off" and compare to zero.  To almost everyone else, the
arrangement seems recondite and unfriendly.

   (5) Hyphenation is prevented if the next page location trap is closer
to the vertical drawing position than the next text baseline would be.
GNU 'troff' automatically inserts an implicit vertical position trap at
the end of each page to cause a page transition.  Users or macro
packages can set such traps explicitly to prevent hyphenation of the
last word in a column in multi-column page layouts or before floating
figures or tables.  *Note Page Location Traps::.

   (6) For more on localization, see the 'groff_tmac(5)' man page.

5.11 Manipulating Spacing
=========================

 -- Request: .sp [distance]
     Space downward by DISTANCE.  Inside a diversion, any DISTANCE
     argument is ignored.  Otherwise, a negative argument moves the
     drawing position up the page.  This request causes a break.  The
     default scaling unit is 'v'.  If DISTANCE is not specified, '1v' is
     assumed.

     You may wish to use the following macros to set the baseline of the
     next output text at a given distance from the top or the bottom of
     the page.  We subtract one line height ('\n[.v]') because the '|'
     operator moves to one vee below the page top (*note Numeric
     Expressions::).

          .de y-from-top-down
          .  sp |\\$1-\\n[.v]u
          ..
          .
          .de y-from-bot-up
          .  sp |\\n[.p]u-\\$1-\\n[.v]u
          ..

     A call to '.y-from-bot-up 10c' means that the next text baseline
     will be at 10 cm from the bottom edge of the paper.

     If a vertical position trap is sprung during execution of 'sp', the
     amount of vertical space after the trap is discarded.

          .de xxx
          ..
          .
          .wh 0 xxx
          .
          .pl 5v
          foo
          .sp 2
          bar
          .sp 50
          baz
              => foo
              =>
              =>
              => bar
              =>
              => baz

     The amount of discarded space is available in the register
     '.trunc'.

     To protect 'sp' against vertical position traps, use the 'vpt'
     request to disable them.  *Note Vertical Position Traps::.

          .vpt 0
          .sp -3
          .vpt 1

 -- Request: .ls [nnn]
 -- Register: \n[.L]
     Output NNN-1 blank lines after each line of text.  With no
     argument, 'gtroff' uses the previous value before the last 'ls'
     call.

          .ls 2    \" This causes double-spaced output
          .ls 3    \" This causes triple-spaced output
          .ls      \" Again double-spaced

     The read-only register '.L' contains the current line spacing
     setting.  The line spacing is associated with the environment
     (*note Environments::).

   *Note Changing the Type Size::, for the requests 'vs' and 'pvs' as
alternatives to 'ls'.

 -- Escape sequence: \x'spacing'
 -- Register: \n[.a]
     Sometimes, extra vertical spacing is needed only occasionally, for
     instance to allow room for a tall construct like an inline
     equation.  The '\x' escape sequence takes a delimited measurement
     (like '\x'3p''); the default scaling unit is 'v'.  If the
     measurement is positive, extra vertical space is inserted below the
     current line; a negative measurement adds space above.  If '\x' is
     used multiple times on the same output line, the maxima of the
     positive and negative adjustments are used.  The delimiter need not
     be a neutral apostrophe; see *note Delimiters::.

     The '.a' read-only register contains the most recent (non-negative)
     extra vertical line space.

     Use of '\x' can be necessary in combination with the '\b' escape
     sequence, as the following example shows.

          This is a test of \[rs]b.
          .br
          This is a test of \[rs]b.
          .br
          This is a test of \b'xyz'\x'-1m'\x'1m'.
          .br
          This is a test of \[rs]b.
          .br
          This is a test of \[rs]b.

     produces

          This is a test of \b.
          This is a test of \b.
                            x
          This is a test of y.
                            z
          This is a test of \b.
          This is a test of \b.

 -- Request: .ns
 -- Request: .rs
 -- Register: \n[.ns]
     Enable "no-space mode".  In this mode, spacing (either via 'sp' or
     via blank lines) is disabled.  The 'bp' request to advance to the
     next page is also disabled, except if it is accompanied by a page
     number (*note Page Control::).  This mode ends when actual text is
     output or the 'rs' request is encountered, which ends no-space
     mode.  The read-only register '.ns' is set to 1 as long as no-space
     mode is active.

     This request is useful for macros that conditionally insert
     vertical space before the text starts (for example, a paragraph
     macro could insert some space except when it is the first paragraph
     after a section header).

5.12 Tabs and Fields
====================

A tab character (ISO code point 9, EBCDIC code point 5) causes a
horizontal movement to the next tab stop, if any.

 -- Escape sequence: \t
     Interpolate a tab in copy mode; see *note Copy Mode::.

 -- Request: .ta [[n1 n2 ... nn ]T r1 r2 ... rn]
 -- Register: \n[.tabs]
     Change tab stop positions.  This request takes a series of tab
     specifiers as arguments (optionally divided into two groups with
     the letter 'T') that indicate where each tab stop is to be,
     overriding any previous settings.  The default scaling unit is 'm'.
     Invoking 'ta' without an argument removes all tab stops.  GNU
     'troff''s startup value is 'T 0.5i'.

     Tab stops can be specified absolutely--as distances from the left
     margin.  The following example sets six tab stops, one every inch.

          .ta 1i 2i 3i 4i 5i 6i

     Tab stops can also be specified using a leading '+', which means
     that the specified tab stop is set relative to the previous tab
     stop.  For example, the following is equivalent to the previous
     example.

          .ta 1i +1i +1i +1i +1i +1i

     GNU 'troff' supports an extended syntax to specify repeating tab
     stops.  These stops appear after a 'T' argument.  Their values are
     always taken as distances relative to the previous tab stop.  This
     is the idiomatic way to specify tab stops at equal intervals in
     'groff'.  The following is, yet again, the same as the previous
     examples.  It does more, in fact, since it defines an infinite
     number of tab stops at one-inch intervals.

          .ta T 1i

     Now we are ready to interpret the full syntax given above.  The
     'ta' request sets tabs at positions N1, N2, ..., NN, then at NN+R1,
     NN+R2, ..., NN+RN, then at NN+RN+R1, NN+RN+R2, ..., NN+RN+RN, and
     so on.

     For example, '4c +6c T 3c 5c 2c' is equivalent to '4c 10c 13c 18c
     20c 23c 28c 30c ...'.

     Text written to a tab column (i.e., between two tab stops, or
     between a tab stop and an output line boundary) may be aligned to
     the right or left, or centered in the column.  This alignment is
     determined by appending 'R', 'L', or 'C' to the tab specifier.  The
     default is 'L'.

          .ta 1i 2iC 3iR

     The beginning of an output line is not a tab stop; the text that
     begins an output line is placed according to the configured
     alignment and indentation; see *note Manipulating Filling and
     Adjustment:: and *note Line Layout::.

     A tab stop is converted into a non-breakable horizontal movement
     that cannot be adjusted.

          .ll 2i
          .ds foo a\tb\tc
          .ta T 1i
          \*[foo]
              error-> warning: cannot break line
              => a         b         c

     The above creates a single output line that is a bit longer than
     two inches (we use a string to show exactly where the tab stops
     are).  Now consider the following.

          .ll 2i
          .ds bar a\tb c\td
          .ta T 1i
          \*[bar]
              error-> warning: cannot adjust line
              => a         b
              => c       d

     GNU 'troff' first converts the line's tab stops into unbreakable
     horizontal movements, then breaks after 'b'.  This usually isn't
     what you want.

     Superfluous tab characters--those that do not correspond to a tab
     stop--are ignored except for the first, which delimits the
     characters belonging to the last tab stop for right-alignment or
     centering.

          .ds Z   foo\tbar\tbaz
          .ds ZZ  foo\tbar\tbazqux
          .ds ZZZ foo\tbar\tbaz\tqux
          .ta 2i 4iR
          \*[Z]
          .br
          \*[ZZ]
          .br
          \*[ZZZ]
          .br
              => foo                 bar              baz
              => foo                 bar           bazqux
              => foo                 bar              bazqux

     The first line right-aligns "baz" within the second tab stop.  The
     second line right-aligns "bazqux" within it.  The third line
     right-aligns only "baz" because of the additional tab character,
     which marks the end of the text occupying the last tab stop
     defined.

     Tab stops are associated with the environment (*note
     Environments::).

     The read-only register '.tabs' contains a string representation of
     the current tab settings suitable for use as an argument to the
     'ta' request.(1)  (*note Tabs and Fields-Footnote-1::)

          .ds tab-string \n[.tabs]
          \*[tab-string]
              => T120u

 -- Request: .tc [fill-glyph]
     Normally, GNU 'troff' writes no glyph when moving to a tab stop
     (some output devices may explicitly output space characters to
     achieve this motion).  A "tab repetition character" can be
     specified with the 'tc' request, causing GNU 'troff' to write as
     many instances of FILL-GLYPH as are necessary to occupy the
     interval from the current horizontal location to the next tab stop.
     With no argument, GNU 'troff' reverts to the default behavior.  The
     tab repetition character is associated with the environment (*note
     Environments::).(2)  (*note Tabs and Fields-Footnote-2::) Only a
     single FILL-GLYPH is recognized; any excess is ignored.

 -- Request: .linetabs n
 -- Register: \n[.linetabs]
     If N is missing or nonzero, activate "line-tabs"; deactivate it
     otherwise (the default).  Active line-tabs cause GNU 'troff' to
     compute tab distances relative to the start of the output line
     instead of the input line.

          .de Tabs
          .  ds x a\t\c
          .  ds y b\t\c
          .  ds z c
          .  ta 1i 3i
          \\*x
          \\*y
          \\*z
          ..
          .Tabs
          .br
          .linetabs
          .Tabs
              => a         b         c
              => a         b                   c

     Line-tabs activation is associated with the environment (*note
     Environments::).  The read-only register '.linetabs' interpolates 1
     if line-tabs are active, and 0 otherwise.

   (1) Plan 9 'troff' uses the register '.S' for this purpose.

   (2) Tab repetition _character_ is a misnomer since it is an output
glyph.

5.12.1 Leaders
--------------

Sometimes it is desirable to fill a tab stop with a given glyph, but
also use tab stops normally on the same output line.  An example is a
table of contents entry that uses dots to bridge the entry name with its
page number, which is itself aligned within a tab stop.  The 'roff'
language provides "leaders" for this purpose.(1)  (*note
Leaders-Footnote-1::)

   A leader character (ISO and EBCDIC code point 1, also known as SOH or
"start of heading"), behaves similarly to a tab character: it moves to
the next tab stop.  The difference is that for this movement, the
default fill glyph is a period '.'.

 -- Escape sequence: \a
     Interpolate a leader in copy mode; see *note Copy Mode::.

 -- Request: .lc [fill-glyph]
     When writing a leader, GNU 'troff' fills the space to the next tab
     stop with dots '.'.  A different "leader repetition character" can
     be specified with the 'lc' request, causing GNU 'troff' to write as
     many instances of FILL-GLYPH as are necessary to occupy the
     interval from the current horizontal location to the next tab stop.
     With no argument, GNU 'troff' treats leaders the same as tabs.  The
     leader repetition character is associated with the environment
     (*note Environments::).(2)  (*note Leaders-Footnote-2::) Only a
     single FILL-GLYPH is recognized; any excess is ignored.

   A table of contents, for example, may define tab stops after a
section number, a title, and a gap to be filled with leader dots.  The
page number follows the leader, after a right-aligned final tab stop
wide enough to house the largest page number occurring in the document.

     .ds entry1 19.\tThe Prophet\a\t98
     .ds entry2 20.\tAll Astir\a\t101
     .ta .5i 4.5i +.5iR
     .nf
     \*[entry1]
     \*[entry2]
         => 19.  The Prophet.............................   98
         => 20.  All Astir...............................  101

   (1) This is pronounced to rhyme with "feeder", and refers to how the
glyphs "lead" the eye across the page to the corresponding page number
or other datum.

   (2) Leader repetition _character_ is a misnomer since it is an output
glyph.

5.12.2 Fields
-------------

"Fields" are a more general way of laying out tabular data.  A field is
defined as the data between a pair of "delimiting characters".  It
contains substrings that are separated by "padding characters".  The
width of a field is the distance on the _input_ line from the position
where the field starts to the next tab stop.  A padding character
inserts an adjustable space similar to TeX's '\hss' command (thus it can
even be negative) to make the sum of all substring lengths plus the
adjustable space equal to the field width.  If more than one padding
character is inserted, the available space is evenly distributed among
them.

 -- Request: .fc [delim-char [padding-char]]
     Define a delimiting and a padding character for fields.  If the
     latter is missing, the padding character defaults to a space
     character.  If there is no argument at all, the field mechanism is
     disabled (which is the default).  In contrast to, e.g., the tab
     repetition character, delimiting and padding characters are _not_
     associated with the environment (*note Environments::).

          .fc # ^
          .ta T 3i
          #foo^bar^smurf#
          .br
          #foo^^bar^smurf#
              => foo         bar          smurf
              => foo            bar       smurf

5.13 Character Translations
===========================

A "translation" is a mapping of an input character to an output glyph.
The mapping occurs at output time, i.e., the input character gets
assigned the metric information of the mapped output character right
before input tokens are converted to nodes (*note Gtroff Internals::,
for more on this process).

 -- Request: .tr abcd...
 -- Request: .trin abcd...
     Translate character A to glyph B, character C to glyph D, and so
     on.  If there is an odd number of characters in the argument, the
     last one is translated to a fixed-width space (the same one
     obtained by the '\ ' escape).

     The 'trin' request is identical to 'tr', but when you unformat a
     diversion with 'asciify' it ignores the translation.  *Note
     Diversions::, for details about the 'asciify' request.

     Some notes:

        * Special characters ('\(XX', '\[XXX]', '\C'XXX'', '\'', '\`',
          '\-', '\_'), glyphs defined with the 'char' request, and
          numbered glyphs ('\N'XXX'') can be translated also.

        * The '\e' escape can be translated also.

        * Characters can be mapped onto the '\%' and '\~' escape
          sequences (but '\%' and '\~' can't be mapped onto another
          glyph).

        * The following characters can't be translated: space (with one
          exception, see below), backspace, newline, leader (and '\a'),
          tab (and '\t').

        * Translations are not considered for finding the soft hyphen
          character set with the 'shc' request.

        * The pair 'C\&' (an arbitrary character C followed by the dummy
          character) maps this character to nothing.

               .tr a\&
               foo bar
                   => foo br

          It is even possible to map the space character to nothing:

               .tr aa \&
               foo bar
                   => foobar

          As shown in the example, the space character can't be the
          first character/glyph pair as an argument of 'tr'.
          Additionally, it is not possible to map the space character to
          any other glyph; requests like '.tr aa x' undo '.tr aa \&'
          instead.

          If justification is active, lines are justified in spite of
          the 'empty' space character (but there is no minimal distance,
          i.e., the space character, between words).

        * After an output glyph has been constructed (this happens at
          the moment immediately before the glyph is appended to an
          output glyph list, either by direct output, in a macro,
          diversion, or string), it is no longer affected by 'tr'.

        * Translating character to glyphs where one of them or both are
          undefined is possible also; 'tr' does not check whether the
          entities in its argument do exist.

          *Note Gtroff Internals::.

        * 'troff' no longer has a hard-coded dependency on Latin-1; all
          'charXXX' entities have been removed from the font description
          files.  This has a notable consequence that shows up in
          warnings like 'can't find character with input code XXX' if
          the 'tr' request isn't handled properly.

          Consider the following translation:

               .tr 

          This maps input character '' onto glyph '', which is
          identical to glyph 'char201'.  But this glyph intentionally
          doesn't exist!  Instead, '\[char201]' is treated as an input
          character entity and is by default mapped onto '\['E]', and
          'gtroff' doesn't handle translations of translations.

          The right way to write the above translation is

               .tr \['E]

          In other words, the first argument of 'tr' should be an input
          character or entity, and the second one a glyph entity.

        * Without an argument, the 'tr' request is ignored.

 -- Request: .trnt abcd...
     'trnt' is the same as the 'tr' request except that the translations
     do not apply to text that is transparently throughput into a
     diversion with '\!'.  *Note Diversions::.

     For example,

          .tr ab
          .di x
          \!.tm a
          .di
          .x

     prints 'b' to the standard error stream; if 'trnt' is used instead
     of 'tr' it prints 'a'.

5.14 'troff' and 'nroff' Modes
==============================

Historically, 'nroff' and 'troff' were two separate programs; the former
for terminal output, the latter for typesetters.  GNU 'troff' merges
both functions into one executable(1) (*note troff and nroff
Modes-Footnote-1::) that sends its output to a device driver ('grotty'
for terminal devices, 'grops' for PostScript, and so on) which
interprets this intermediate output format.  When discussing AT&T
'troff', it makes sense to talk about "'nroff' mode" and "'troff' mode"
since the differences are hard-coded.  GNU 'troff' takes information
from device and font description files without handling requests
specially if a terminal output device is used, so such a strong
distinction is unnecessary.

   Usually, a macro package can be used with all output devices.
Nevertheless, it is sometimes necessary to make a distinction between
terminal and non-terminal devices: GNU 'troff' provides two built-in
conditions 'n' and 't' for the 'if', 'ie', and 'while' requests to
decide whether GNU 'troff' shall behave like 'nroff' or like 'troff'.

 -- Request: .troff
     Make the 't' built-in condition true (and the 'n' built-in
     condition false) for 'if', 'ie', and 'while' conditional requests.
     This is the default if GNU 'troff' (_not_ 'groff') is started with
     the '-R' switch to avoid loading of the startup files 'troffrc' and
     'troffrc-end'.  Without '-R', GNU 'troff' stays in 'troff' mode if
     the output device is not a terminal (e.g., 'ps').

 -- Request: .nroff
     Make the 'n' built-in condition true (and the 't' built-in
     condition false) for 'if', 'ie', and 'while' conditional requests.
     This is the default if GNU 'troff' uses a terminal output device;
     the code for switching to 'nroff' mode is in the file 'tty.tmac',
     which is loaded by the startup file 'troffrc'.

   *Note Conditionals and Loops::, for more details on built-in
conditions.

   (1) A GNU 'nroff' program is available for convenience; it calls GNU
'troff' to perform the formatting.

5.15 Line Layout
================

The following drawing shows the dimensions that 'gtroff' uses for
placing a line of output onto the page.  They are labeled with the
request that manipulates each dimension.

          -->| in |<--
             |<-----------ll------------>|
        +----+----+----------------------+----+
        |    :    :                      :    |
        +----+----+----------------------+----+
     -->| po |<--
        |<--------paper width---------------->|

These dimensions are:

'po'
     "Page offset"--this is the leftmost position of text on the final
     output, defining the "left margin".

'in'
     "Indentation"--this is the distance from the left margin where text
     is printed.

'll'
     "Line length"--this is the distance from the left margin to right
     margin.

   The right margin is not explicitly configured; the combination of
page offset and line length provides the information necessary to derive
it.

   A simple demonstration:

     .ll 3i
     This is text without indentation.
     The line length has been set to 3\~inches.
     .in +.5i
     .ll -.5i
     Now the left and right margins are both increased.
     .in
     .ll
     Calling .in and .ll without parameters restores
     the previous values.

   Result:

     This  is text without indenta-
     tion.   The  line  length  has
     been set to 3 inches.
          Now   the  left  and
          right  margins   are
          both increased.
     Calling  .in  and  .ll without
     parameters restores the previ-
     ous values.

 -- Request: .po [offset]
 -- Request: .po +offset
 -- Request: .po -offset
 -- Register: \n[.o]
     Set page offset to OFFSET (or increment or decrement its current
     value by OFFSET).  If invoked without an argument, the page offset
     is restored to the value before the previous 'po' request.  This
     request does not cause a break; the page offset in effect when an
     output line is broken prevails (*note Manipulating Filling and
     Adjustment::).  The initial value is 1i and the default scaling
     unit is 'm'.  On terminal devices, the page offset is set to zero
     by a driver-specific macro file, 'tty.tmac'.  The current page
     offset can be found in the read-only register '.o'.  This request
     is incorrectly documented in the AT&T 'troff' manual as using a
     default scaling unit of 'v'.

          .po 3i
          \n[.o]
              => 720
          .po -1i
          \n[.o]
              => 480
          .po
          \n[.o]
              => 720

 -- Request: .in [indent]
 -- Request: .in +indent
 -- Request: .in -indent
 -- Register: \n[.i]
     Set indentation to INDENT (or increment or decrement the current
     value by INDENT).  This request causes a break.  Initially, there
     is no indentation.

     If 'in' is called without an argument, the indentation is reset to
     the previous value before the last call to 'in'.  The default
     scaling indicator is 'm'.

     If a negative indentation value is specified (which is not
     allowed), 'gtroff' emits a warning in category 'range' and sets the
     indentation to zero.

     The effect of 'in' is delayed until a partially collected line (if
     it exists) is output.  A temporary indentation value is reset to
     zero also.

     The current indentation (as set by 'in') can be found in the
     read-only register '.i'.  The indentation is associated with the
     environment (*note Environments::).

 -- Request: .ti offset
 -- Request: .ti +offset
 -- Request: .ti -offset
 -- Register: \n[.in]
     Temporarily indent the next output line by OFFSET.  If an increment
     or decrement value is specified, adjust the temporary indentation
     relative to the value set by the 'in' request.

     This request causes a break; its value is associated with the
     environment (*note Environments::).  The default scaling indicator
     is 'm'.  A call of 'ti' without an argument is ignored.

     If the total indentation value is negative (which is not allowed),
     'gtroff' emits a warning in category 'range' and sets the temporary
     indentation to zero.  'Total indentation' is either OFFSET if
     specified as an absolute value, or the temporary plus normal
     indentation, if OFFSET is given as a relative value.

     The effect of 'ti' is delayed until a partially collected line (if
     it exists) is output.

     The read-only register '.in' is the indentation that applies to the
     current output line.

     The difference between '.i' and '.in' is that the latter takes into
     account whether a partially collected line still uses the old
     indentation value or a temporary indentation value is active.

 -- Request: .ll [length]
 -- Request: .ll +length
 -- Request: .ll -length
 -- Register: \n[.l]
 -- Register: \n[.ll]
     Set the line length to LENGTH (or increment or decrement the
     current value by LENGTH).  Initially, the line length is set to
     6.5i.  The effect of 'll' is delayed until a partially collected
     line (if it exists) is output.  The default scaling indicator is
     'm'.

     If 'll' is called without an argument, the line length is reset to
     the previous value before the last call to 'll'.  If a negative
     line length is specified (which is not allowed), 'gtroff' emits a
     warning in category 'range' and sets the line length to zero.  The
     line length is associated with the environment (*note
     Environments::).

     The current line length (as set by 'll') can be found in the
     read-only register '.l'.  The read-only register '.ll' is the line
     length that applies to the current output line.

     Similar to '.i' and '.in', the difference between '.l' and '.ll' is
     that the latter takes into account whether a partially collected
     line still uses the old line length value.

5.16 Line Continuation
======================

When filling is enabled, input and output line breaks generally do not
correspond.  The 'roff' language therefore distinguishes input and
output line continuation.

 -- Escape sequence: \<RET>
     '\<RET>' (a backslash immediately followed by a newline) suppresses
     the effects of that newline in the input.  The next input line thus
     retains the classification of its predecessor as a control or text
     line.  '\<RET>' is useful for managing line lengths in the input
     during document maintenance; you can break an input line in the
     middle of a request invocation, macro call, or escape sequence.
     Input line continuation is invisible to the formatter, with two
     exceptions: the '|' operator recognizes the new input line (*note
     Numeric Expressions::), and the input line counter register '.c' is
     incremented.

          .ll 50n
          .de I
          .  ft I
          .  nop \\$*
          .  ft
          ..
          Our film class watched
          .I The Effect of Gamma Rays on Man-in-the-Moon
          Marigolds. \" whoops, the input line wrapped
          .br
          .I My own opus begins on line \n[.c] \
          and ends on line \n[.c].
              => Our film class watched The Effect of Gamma Rays on
              => Man-in-the-Moon Marigolds.
              => My own opus begins on line 11 and ends on line 12.

 -- Escape sequence: \c
 -- Register: \n[.int]
     '\c' continues an output line.  Nothing on the input line after it
     is formatted.  In contrast to '\<RET>', a line after '\c' is
     treated as a new input line, so a control character is recognized
     at its beginning.  The visual results depend on whether filling is
     enabled; see *note Manipulating Filling and Adjustment::.

        * If filling is enabled, a word interrupted with '\c' is
          continued with the text on the next input text line, without
          an intervening space.

               This is a te\c
               st.
                   => This is a test.

        * If filling is disabled, the next input text line after '\c' is
          handled as a continuation of the same input text line.

               .nf
               This is a \c
               test.
                   => This is a test.

     An intervening control line that causes a break overrides '\c',
     flushing out the pending output line in the usual way.

     The '.int' register contains a positive value if the last output
     line was continued with '\c'; this datum is associated with the
     environment (*note Environments::).(1)  (*note Line
     Continuation-Footnote-1::)

   (1) Historically, the '\c' escape sequence has proven challenging to
characterize.  Some sources say it "connects the next input text" (to
the input line on which it appears); others describe it as
"interrupting" text, on the grounds that a text line is interrupted
without breaking, perhaps to inject a request invocation or macro call.

5.17 Page Layout
================

GNU 'troff' provides some primitive operations for controlling page
layout.

 -- Request: .pl [length]
 -- Request: .pl +length
 -- Request: .pl -length
 -- Register: \n[.p]
     Set the "page length" to LENGTH (or increment or decrement the
     current value by LENGTH).  This is the length of the physical
     output page.  The default scaling indicator is 'v'.

     The current setting can be found in the read-only register '.p'.

     This specifies only the size of the page, not the top and bottom
     margins.  Those are not set by GNU 'troff' directly.  *Note
     Traps::, for further information on how to do this.

     Negative 'pl' values are possible also, but not very useful: no
     trap is sprung, and each line is output on a single page (thus
     suppressing all vertical spacing).

     If no argument or an invalid argument is given, 'pl' sets the page
     length to 11i.

   GNU 'troff' provides several operations that help in setting up top
and bottom titles (also known as headers and footers).

 -- Request: .tl 'left'center'right'
     Print a "title line".  It consists of three parts: a left-justified
     portion, a centered portion, and a right-justified portion.  The
     argument separator ''' can be replaced with any character not
     occurring in the title line.  The '%' character is replaced with
     the current page number.  This character can be changed with the
     'pc' request (see below).  The delimiter need not be a neutral
     apostrophe: 'tl' accepts the same delimiters as most escape
     sequences; see *note Delimiters::.  Without an argument, 'tl' is
     ignored.

        * The line length set by the 'll' request is not honoured by
          'tl'; use the 'lt' request (described below) instead, to
          control line length for text set by 'tl'.

        * A title line is not restricted to the top or bottom of a page.

        * 'tl' prints the title line immediately, ignoring a partially
          collected line (which stays untouched).

        * It is not an error to omit closing delimiters.  For example,
          '.tl /foo' is equivalent to '.tl /foo///': It prints a title
          line with the left-justified word 'foo'; the centered and
          right-justified parts are empty.

 -- Request: .lt [length]
 -- Request: .lt +length
 -- Request: .lt -length
 -- Register: \n[.lt]
     The title line is printed using its own line length, which is
     specified (or incremented or decremented) with the 'lt' request.
     Initially, the title line length is set to 6.5i.  If a negative
     line length is specified (which is not allowed), 'gtroff' emits a
     warning in category 'range' and sets the title line length to zero.
     The default scaling indicator is 'm'.  If 'lt' is called without an
     argument, the title length is reset to the previous value before
     the last call to 'lt'.  The current setting is available in the
     '.lt' read-only register; it is associated with the environment
     (*note Environments::).

 -- Request: .pn page
 -- Request: .pn +page
 -- Request: .pn -page
 -- Register: \n[.pn]
     Change (increase or decrease) the page number of the _next_ page.
     The only argument is the page number; the request is ignored
     without a parameter.

     The read-only register '.pn' contains the number of the next page:
     either the value set by a 'pn' request, or the number of the
     current page plus 1.

 -- Request: .pc [char]
     Change the page number character (used by the 'tl' request) to a
     different character.  With no argument, this mechanism is disabled.
     This doesn't affect the register '%'.

   *Note Traps::.

5.18 Page Control
=================

 -- Request: .bp [page]
 -- Request: .bp +page
 -- Request: .bp -page
 -- Register: \n[%]
     Stop processing the current page and move to the next page.  This
     request causes a break.  It can also take an argument to set
     (increase, decrease) the page number of the next page (which
     becomes the current page after 'bp' has finished).  The difference
     between 'bp' and 'pn' is that 'pn' does not cause a break or
     actually eject a page.  *Note Page Layout::.  This request is
     incorrectly documented in the AT&T 'troff' manual as having a
     default scaling indicator of 'v'.

          .de newpage                         \" define macro
          'bp                                 \" begin page
          'sp .5i                             \" vertical space
          .tl 'left top'center top'right top' \" title
          'sp .3i                             \" vertical space
          ..                                  \" end macro

     'bp' has no effect if not called within the top-level diversion
     (*note Diversions::).

     The writable register '%' holds the current page number.

     The register '.pe' is set to 1 while 'bp' is active.  *Note Page
     Location Traps::.

 -- Request: .ne [space]
     Your text may need a certain amount of vertical space before a page
     break occurs.  For instance, you may wish to ensure that the first
     output line of a paragraph is not "orphaned" at the bottom of a
     page.  The 'ne' request tests the amount of distance to the next
     page location trap (or the page bottom if none is planted earlier;
     see *note Page Location Traps::), and breaks the page if less than
     SPACE is available.  The default scaling unit is 'v'.  If SPACE is
     not specified, '1v' is assumed.

     For example, to require room for at least the first two output
     lines of a paragraph, you can do the following.

          .ne 2v
          Considering how common illness is,
          how tremendous the spiritual change that it brings,
          how astonishing,
          when the lights of health go down,
          the undiscovered countries that are then disclosed,

     This method is reliable only if no output line is pending when 'ne'
     is invoked.  When macro packages are used, this is often not the
     case: their paragraphing macros perform the break.  You may need to
     experiment with placing the 'ne' after the paragraphing macro, or
     'br' and 'ne' before it.

     'ne' is also useful to force grouping of section headings with
     their subsequent paragraphs, or tables with their captions and/or
     explanations.  Macro packages often use 'ne' with diversions to
     implement keeps and displays; see *note Diversions::.  They may
     also offer parameters for widow and orphan management.

 -- Request: .sv [space]
 -- Request: .os
     The 'sv' request is similar to 'ne', but saves the specified
     vertical space.  If SPACE is available before the next page
     location trap (or the page bottom if none is planted earlier; see
     *note Page Location Traps::), the space is output immediately.  Any
     partially collected line is ignored.  Otherwise, the vertical space
     is saved.  Output the space on demand with 'os'.  Both 'sv' and
     'os' ignore no-space mode (recall *note Manipulating Spacing::).
     While the 'sv' request allows negative values for SPACE, 'os'
     ignores them.  The default scaling unit is 'v'.  If SPACE is not
     specified, '1v' is assumed.

 -- Register: \n[nl]
     'nl' interpolates or sets the vertical drawing position.  When the
     formatter starts and first page transition hasn't happened yet,
     'nl' is negative.  If a header trap has been planted on the page
     (typically at vertical position '0'), you can assign a negative
     value to 'nl' to spring it if that page has already started (*note
     Page Location Traps::).

          .de header
          .  sp
          .  tl ''Goldbach Solution''
          .  sp
          ..
          .
          First page.
          ...
          .bp
          .wh 0 header \" plant header trap at top of page
          .nr nl (-1)
          Second page.
          ...
              => First page.
              =>
              => ...
              =>
              =>                   Goldbach Solution
              =>
              => Second page.
              =>
              => ...

     Without resetting 'nl' to a negative value, the trap just planted
     would be active beginning with the _next_ page, not the current
     one.

     *Note Diversions::, for a comparison of 'nl' with the '.h' and '.d'
     registers.

5.19 Fonts and Symbols
======================

'gtroff' can switch fonts at any point in the text.

   The basic set of fonts is 'R', 'I', 'B', and 'BI'.  These are Times
roman, italic, bold, and bold-italic.  For non-terminal devices, there
is also at least one symbol font that contains various special symbols
(Greek, mathematics).

5.19.1 Changing Fonts
---------------------

 -- Request: .ft [font]
 -- Escape sequence: \ff
 -- Escape sequence: \f(fn
 -- Escape sequence: \f[font]
 -- Register: \n[.sty]
     The 'ft' request and the '\f' escape change the current font to
     FONT (one-character name F, two-character name FN).

     If FONT is a style name (as set with the 'sty' request or with the
     'styles' command in the 'DESC' file), use it within the current
     font family (as set with the 'fam' request, the '\F' escape, or the
     'family' command in the 'DESC' file).

     It is not possible to switch to a font with the name 'DESC'
     (whereas this name could be used as a style name; however, this is
     not recommended).

     With no argument or using 'P' as an argument, 'ft' switches to the
     previous font.  Use '\f[]' to do this with an escape sequence.  The
     old syntax forms '\fP' or '\f[P]' are also supported.

     Fonts are generally specified as uppercase strings, which are
     usually 1 to 4 characters representing an abbreviation or acronym
     of the font name.  This is no limitation, just a convention.

     The example below produces two identical lines.

          eggs, bacon,
          .ft B
          spam
          .ft
          and sausage.

          eggs, bacon, \fBspam\fP and sausage.

     '\f' doesn't produce an input token in GNU 'troff'.  As a
     consequence, it can be used in requests like 'mc' (which expects a
     single character as an argument) to change the font on the fly:

          .mc \f[I]x\f[]

     The current style name is available in the read-only string-valued
     register '.sty'; it is associated with the environment (*note
     Environments::).  If the current font isn't a style, interpolating
     '.sty' produces nothing.

     *Note Font Positions::, for an alternative syntax.

 -- Request: .ftr f [g]
     Translate font F to font G.  Whenever a font named F is referred to
     in a '\f' escape sequence, in the 'F' and 'S' conditional
     operators, or in the 'ft', 'ul', 'bd', 'cs', 'tkf', 'special',
     'fspecial', 'fp', or 'sty' requests, font G is used.  If G is
     missing or equal to F the translation is undone.

     Font translations cannot be chained.

          .ftr XXX TR
          .ftr XXX YYY
          .ft XXX
              error-> warning: can't find font 'XXX'

 -- Request: .fzoom f [zoom]
 -- Register: \n[.zoom]
     Set magnification of font F to factor ZOOM, which must be a
     non-negative integer multiple of 1/1000th.  This request is useful
     to adjust the optical size of a font in relation to the others.  In
     the example below, font 'CR' is magnified by 10% (the zoom factor
     is thus 1.1).

          .fam P
          .fzoom CR 1100
          .ps 12
          Palatino and \f[CR]Courier\f[]

     A missing or zero value of ZOOM is the same as a value of 1000,
     which means no magnification.  F must be a real font name, not a
     style.

     The magnification of a font is completely transparent to GNU
     'troff'; a change of the zoom factor doesn't cause any effect
     except that the dimensions of glyphs, (word) spaces, kerns, etc.,
     of the affected font are adjusted accordingly.

     The zoom factor of the current font is available in the read-only
     register '.zoom', in multiples of 1/1000th.  It returns zero if
     there is no magnification.

5.19.2 Font Families
--------------------

Due to the variety of fonts available, 'gtroff' has added the concept of
"font families" and "font styles".  The fonts are specified as the
concatenation of the font family and style.  Specifying a font without
the family part causes 'gtroff' to use that style of the current family.

   Currently, fonts for the devices '-Tps', '-Tpdf', '-Tdvi', '-Tlj4',
'-Tlbp', and the X11 fonts are set up to this mechanism.  By default,
'gtroff' uses the Times family with the four styles 'R', 'I', 'B', and
'BI'.

   This way, it is possible to use the basic four fonts and to select a
different font family on the command line (*note Groff Options::).

 -- Request: .fam [family]
 -- Register: \n[.fam]
 -- Escape sequence: \Ff
 -- Escape sequence: \F(fm
 -- Escape sequence: \F[family]
 -- Register: \n[.fn]
     Set the font family to FAMILY (one-character name F, two-character
     name FM).  If no argument is given, switch to the previous font
     family, or the default family if there is none.  Use '\F[]' to do
     this with an escape sequence; '\FP' selects font family 'P'
     instead.  The initial font family is 'T' (Times), but can be
     overridden by the output device description file--*Note DESC File
     Format::.  The current font family is available in the read-only
     string-valued register '.fam'; it is associated with the
     environment (*note Environments::).

          spam,
          .fam H    \" helvetica family
          spam,     \" used font is family H + style R = HR
          .ft B     \" family H + style B = font HB
          spam,
          .fam T    \" times family
          spam,     \" used font is family T + style B = TB
          .ft AR    \" font AR (not a style)
          baked beans,
          .ft R     \" family T + style R = font TR
          and spam.

     '\F' doesn't produce an input token in GNU 'troff'.  As a
     consequence, it can be used in requests like 'mc' (which expects a
     single character as an argument) to change the font family on the
     fly.

          .mc \F[P]x\F[]

     The read-only string-valued register '.fn' contains the current
     "real font name" of the current font.  If the current font is a
     style, the value of '\n[.fn]' is the proper concatenation of family
     and style name.

 -- Request: .sty n style
     Associate STYLE with font position N.  A font position can be
     associated either with a font or with a style.  The current font is
     the index of a font position and so is also either a font or a
     style.  If it is a style, the font that is actually used is the
     font whose name is the concatenation of the name of the current
     family and the name of the current style.  For example, if the
     current font is 1 and font position 1 is associated with style 'R'
     and the current font family is 'T', then font 'TR' is used.  If the
     current font is not a style, then the current family is ignored.
     If the requests 'cs', 'bd', 'tkf', 'uf', or 'fspecial' are applied
     to a style, they are instead applied to the member of the current
     family corresponding to that style.

     N must be a non-negative integer.

     The default family can be set with the '-f' option (*note Groff
     Options::).  The 'styles' command in the 'DESC' file controls which
     font positions (if any) are initially associated with styles rather
     than fonts.  For example, the default setting for PostScript fonts

          styles R I B BI

     is equivalent to

          .sty 1 R
          .sty 2 I
          .sty 3 B
          .sty 4 BI

     'fam' and '\F' always check whether the current font position is
     valid; this can give surprising results if the current font
     position is associated with a style.

     In the following example, we want to access the PostScript font
     'FooBar' from the font family 'Foo':

          .sty \n[.fp] Bar
          .fam Foo
              error-> warning: can't find font 'FooR'

     The default font position at startup is 1; for the PostScript
     device, this is associated with style 'R', so 'gtroff' tries to
     open 'FooR'.

     A solution to this problem is to use a dummy font like the
     following:

          .fp 0 dummy TR    \" set up dummy font at position 0
          .sty \n[.fp] Bar  \" register style `Bar'
          .ft 0             \" switch to font at position 0
          .fam Foo          \" activate family `Foo'
          .ft Bar           \" switch to font `FooBar'

     *Note Font Positions::.

5.19.3 Font Positions
---------------------

To support typeface indirection through styles, and for compatibility
with AT&T 'troff', the formatter maintains a list of font "positions" at
which fonts required by a document are "mounted".  An output device's
description file 'DESC' typically configures a set of pre-mounted fonts;
see *note Device and Font Description Files::.  A font need not be
explicitly mounted before it is selected; GNU 'troff' will search
'GROFF_FONT_PATH' for it by name and mount it at the first free mounting
position on demand.

 -- Request: .fp pos font [external-name]
 -- Register: \n[.f]
 -- Register: \n[.fp]
     Mount the font named FONT at position POS, a non-negative integer.
     This numeric index can then be referred to with font selection
     instructions.  When the formatter starts up, it reads the output
     device's description to mount an initial set of faces, and selects
     font position 1.  Position 0 is unused by default.  Unless the
     EXTERNAL-NAME argument is given, the name FONT should be that of a
     font description file stored in a directory corresponding to the
     selected output device.

     The position of the currently selected font (or style) is available
     in the read-only register '.f'.  It is associated with the
     environment (*note Environments::).

     The value of '.f' can be copied to another register to save the
     current font or style for later recall.

          .nr saved-font \n[.f]
          .ft B
          ... text text text ...
          .ft \n[saved-font]

     The index of the next (nonzero) free font position is available in
     the read-only register '.fp'.  Fonts not listed in the 'DESC' file
     are automatically mounted at position '\n[.fp]' when selected.
     When mounting a font at a position explicitly with the 'fp'
     request, this same practice should be followed, although GNU
     'troff' does not enforce this strictly.

     If there is no third argument, the font description is sought in
     the file system under the name FONT.

     The optional third argument EXTERNAL-NAME is the name of a font
     description file in the operating environment.  GNU 'troff' refuses
     to traverse directories to locate the font description.  The second
     argument FONT is then the internal name of the font, which
     identifies it to GNU 'troff' once it has been mounted.  This
     feature enables font names to be aliased, which can be necessary in
     compatibility mode since AT&T 'troff' syntax affords no means of
     identifying fonts with names longer than two characters, like 'TBI'
     or 'ZCMI', in a font selection escape sequence.  *Note
     Compatibility Mode::.  You can also alias fonts on mounting for
     convenience or abstraction.

          .ft SC ZCMI \" "script" font
          Send a \f(SChand-written\fP thank-you note.
          .ft Emph I
          .ft Strong B
          Are \f[Emph]these names\f[] \f[Strong]comfortable\f[]?

   The 'ft' request and '\f' escape sequence accept mounting positions
in the place of font or style names.

 -- Request: .ft nnn
 -- Escape sequence: \fn
 -- Escape sequence: \f(nn
 -- Escape sequence: \f[nnn]
     Select font position NNN (one-digit position N, two-digit position
     NN), which must be a non-negative integer.  If NNN is associated
     with a style (as set with the 'sty' request or with the 'styles'
     command in the 'DESC' file), use it within the current font family
     (as set with the 'fam' request, the '\F' escape sequence, or the
     'family' command in the 'DESC' file).

          this is font 1
          .ft 2
          this is font 2
          .ft                   \" switch back to font 1
          .ft 3
          this is font 3
          .ft
          this is font 1 again

     *Note Changing Fonts::, for font selection by name.

5.19.4 Using Symbols
--------------------

A "glyph" is a graphical representation of a "character".  While a
character is an abstract entity containing semantic information, a glyph
is something that can be actually seen on screen or paper.  It is
possible that a character has multiple glyph representation forms (for
example, the character 'A' can be either written in a roman or an italic
font, yielding two different glyphs); sometimes more than one character
maps to a single glyph (this is a "ligature"--the most common is 'fi').

   A "symbol" is simply a named glyph.  Within 'gtroff', all glyph names
of a particular font are defined in its font file.  If the user requests
a glyph not available in this font, 'gtroff' looks up an ordered list of
"special fonts".  By default, the PostScript output device supports the
two special fonts 'SS' (slanted symbols) and 'S' (symbols) (the former
is looked up before the latter).  Other output devices use different
names for special fonts.  Fonts mounted with the 'fonts' keyword in the
'DESC' file are globally available.  To install additional special fonts
locally (i.e., for a particular font), use the 'fspecial' request.

   Here are the exact rules how 'gtroff' searches a given symbol:

   * If the symbol has been defined with the 'char' request, use it.
     This hides a symbol with the same name in the current font.

   * Check the current font.

   * If the symbol has been defined with the 'fchar' request, use it.

   * Check whether the current font has a font-specific list of special
     fonts; test all fonts in the order of appearance in the last
     'fspecial' call if appropriate.

   * If the symbol has been defined with the 'fschar' request for the
     current font, use it.

   * Check all fonts in the order of appearance in the last 'special'
     call.

   * If the symbol has been defined with the 'schar' request, use it.

   * As a last resort, consult all fonts loaded up to now for special
     fonts and check them, starting with the lowest font number.  This
     can sometimes lead to surprising results since the 'fonts' line in
     the 'DESC' file often contains empty positions, which are filled
     later on.  For example, consider the following:

          fonts 3 0 0 FOO

     This mounts font 'foo' at font position 3.  We assume that 'FOO' is
     a special font, containing glyph 'foo', and that no font has been
     loaded yet.  The line

          .fspecial BAR BAZ

     makes font 'BAZ' special only if font 'BAR' is active.  We further
     assume that 'BAZ' is really a special font, i.e., the font
     description file contains the 'special' keyword, and that it also
     contains glyph 'foo' with a special shape fitting to font 'BAR'.
     After executing 'fspecial', font 'BAR' is loaded at font
     position 1, and 'BAZ' at position 2.

     We now switch to a new font 'XXX', trying to access glyph 'foo'
     that is assumed to be missing.  There are neither font-specific
     special fonts for 'XXX' nor any other fonts made special with the
     'special' request, so 'gtroff' starts the search for special fonts
     in the list of already mounted fonts, with increasing font
     positions.  Consequently, it finds 'BAZ' before 'FOO' even for
     'XXX', which is not the intended behaviour.

   *Note Device and Font Description Files::, and *note Special Fonts::,
for more details.

   The list of available symbols is device dependent; see the
'groff_char(7)' man page for a complete list of all glyphs.  For
example, say

     man -Tdvi groff_char > groff_char.dvi

for a list using the default DVI fonts (not all versions of the 'man'
program support the '-T' option).  If you want to use an additional
macro package to change the used fonts, 'groff' must be called directly:

     groff -Tdvi -mec -man groff_char.7 > groff_char.dvi

   Glyph names not listed in 'groff_char(7)' are derived
algorithmically, using a simplified version of the Adobe Glyph List
(AGL) algorithm, which is described in
<https://github.com/adobe-type-tools/agl-aglfn>.  The (frozen) set of
glyph names that can't be derived algorithmically is called the "'groff'
glyph list (GGL)".

   * A glyph for Unicode character U+XXXX[X[X]], which is not a
     composite character is named 'uXXXX[X[X]]'.  X must be an uppercase
     hexadecimal digit.  Examples: 'u1234', 'u008E', 'u12DB8'.  The
     largest Unicode value is 0x10FFFF. There must be at least four 'X'
     digits; if necessary, add leading zeroes (after the 'u').  No zero
     padding is allowed for character codes greater than 0xFFFF.
     Surrogates (i.e., Unicode values greater than 0xFFFF represented
     with character codes from the surrogate area U+D800-U+DFFF) are not
     allowed either.

   * A glyph representing more than a single input character is named

          'u' COMPONENT1 '_' COMPONENT2 '_' COMPONENT3 ...

     Example: 'u0045_0302_0301'.

     For simplicity, all Unicode characters that are composites must be
     maximally decomposed to NFD;(1) (*note Using Symbols-Footnote-1::)
     for example, 'u00CA_0301' is not a valid glyph name since U+00CA
     (LATIN CAPITAL LETTER E WITH CIRCUMFLEX) can be further decomposed
     into U+0045 (LATIN CAPITAL LETTER E) and U+0302 (COMBINING
     CIRCUMFLEX ACCENT).  'u0045_0302_0301' is thus the glyph name for
     U+1EBE, LATIN CAPITAL LETTER E WITH CIRCUMFLEX AND ACUTE.

   * groff maintains a table to decompose all algorithmically derived
     glyph names that are composites itself.  For example, 'u0100'
     (LATIN LETTER A WITH MACRON) is automatically decomposed into
     'u0041_0304'.  Additionally, a glyph name of the GGL is preferred
     to an algorithmically derived glyph name; 'groff' also
     automatically does the mapping.  Example: The glyph 'u0045_0302' is
     mapped to '^E'.

   * glyph names of the GGL can't be used in composite glyph names; for
     example, '^E_u0301' is invalid.

 -- Escape sequence: \(nm
 -- Escape sequence: \[name]
 -- Escape sequence: \[base-glyph combining-component ...]
     Typeset a special character NAME (two-character name NM) or a
     composite glyph consisting of BASE-GLYPH overlaid with one or more
     COMBINING-COMPONENTs.  For example, '\[A ho]' is a capital letter
     "A" with a "hook accent" (ogonek).

     There is no special syntax for one-character names--the analogous
     form '\N' would collide with other escape sequences.  However, the
     four escape sequences '\'', '\-', '\_', and '\`', are translated on
     input to the special character escape sequences '\[aa]', '\[-]',
     '\[ul]', and '\[ga]', respectively.

     A special character name of length one is not the same thing as an
     ordinary input character: that is, the character 'a' is not the
     same as '\[a]'.

     If NAME is undefined, a warning in category 'char' is produced and
     the escape is ignored.  *Note Warnings::, for information about the
     enablement and suppression of warnings.

     GNU 'troff' resolves '\[...]' with more than a single component as
     follows:

        * Any component that is found in the GGL is converted to the
          'uXXXX' form.

        * Any component 'uXXXX' that is found in the list of
          decomposable glyphs is decomposed.

        * The resulting elements are then concatenated with '_' in
          between, dropping the leading 'u' in all elements but the
          first.

     No check for the existence of any component (similar to 'tr'
     request) is done.

     Examples:

     '\[A ho]'
          'A' maps to 'u0041', 'ho' maps to 'u02DB', thus the final
          glyph name would be 'u0041_02DB'.  Note this is not the
          expected result: The ogonek glyph 'ho' is a spacing ogonek,
          but for a proper composite a non-spacing ogonek (U+0328) is
          necessary.  Looking into the file 'composite.tmac' one can
          find '.composite ho u0328', which changes the mapping of 'ho'
          while a composite glyph name is constructed, causing the final
          glyph name to be 'u0041_0328'.

     '\[^E u0301]'
     '\[^E aa]'
     '\[E a^ aa]'
     '\[E ^ ']'
          '^E' maps to 'u0045_0302', thus the final glyph name is
          'u0045_0302_0301' in all forms (assuming proper calls of the
          'composite' request).

     It is not possible to define glyphs with names like 'A ho' within a
     'groff' font file.  This is not really a limitation; instead, you
     have to define 'u0041_0328'.

 -- Escape sequence: \C'xxx'
     Typeset the glyph named XXX.(2)  (*note Using Symbols-Footnote-2::)
     Normally it is more convenient to use '\[XXX]', but '\C' has the
     advantage that it is compatible with newer versions of AT&T 'troff'
     and is available in compatibility mode.

 -- Request: .composite from to
     Map glyph name FROM to glyph name TO if it is used in '\[...]' with
     more than one component.  See above for examples.

     This mapping is based on glyph names only; no check for the
     existence of either glyph is done.

     A set of default mappings for many accents can be found in the file
     'composite.tmac', which is loaded at startup.

 -- Escape sequence: \N'n'
     Typeset the glyph with code N in the current font ('n' is _not_ the
     input character code).  The number N can be any non-negative
     decimal integer.  Most devices only have glyphs with codes between
     0 and 255; the Unicode output device uses codes in the range
     0-65535.  If the current font does not contain a glyph with that
     code, special fonts are _not_ searched.  The '\N' escape sequence
     can be conveniently used in conjunction with the 'char' request:

          .char \[phone] \f[ZD]\N'37'

     The code of each glyph is given in the fourth column in the font
     description file after the 'charset' command.  It is possible to
     include unnamed glyphs in the font description file by using a name
     of '---'; the '\N' escape sequence is the only way to use these.

     No kerning is applied to glyphs accessed with '\N'.

   Some escape sequences directly map onto special glyphs.

 -- Escape sequence: \'
     A backslash followed by the apostrophe character, ASCII character
     '0x27' (EBCDIC character '0x7D'), is a synonym for '\[aa]', the
     acute accent.

 -- Escape sequence: \`
     A backslash followed by ASCII character '0x60' (EBCDIC character
     '0x79' [usually]), is a synonym for '\[ga]', the grave accent.

 -- Escape sequence: \-
     A backslash followed by a dash is a synonym for '\[-]', the minus
     sign.

 -- Escape sequence: \_
     A backslash followed by an underscore is a synonym for '\[ul]', the
     underrule.  On 'troff'-mode (typesetter) devices it may be
     font-invariant and drawn lower than the underscore glyph '_'.

 -- Request: .cflags n c1 c2 ...
     Assign properties encoded by the number N to characters C1, C2, and
     so on.

     Input characters, including special characters introduced by an
     escape, have certain properties associated with them.(3)  (*note
     Using Symbols-Footnote-3::) These properties can be modified with
     this request.  The first argument is the sum of the desired flags
     and the remaining arguments are the characters to be assigned those
     properties.  Spaces between the CN arguments are optional.  Any
     argument CN can be a character class defined with the 'class'
     request rather than an individual character.  *Note Character
     Classes::.

     The non-negative integer N is the sum of any of the following.
     Some combinations are nonsensical, such as '33' (1 + 32).

     '1'
          Recognize the character as ending a sentence if followed by a
          newline or two spaces.  Initially, characters '.?!' have this
          property.

     '2'
          Enable breaks before the character.  A line is not broken at a
          character with this property unless the characters on each
          side both have non-zero hyphenation codes.  This exception can
          be overridden by adding 64.  Initially, no characters have
          this property.

     '4'
          Enable breaks after the character.  A line is not broken at a
          character with this property unless the characters on each
          side both have non-zero hyphenation codes.  This exception can
          be overridden by adding 64.  Initially, characters
          '\-\[hy]\[em]' have this property.

     '8'
          Mark the glyph associated with this character as overlapping
          other instances of itself horizontally.  Initially, characters
          '\[ul]\[rn]\[ru]\[radicalex]\[sqrtex]' have this property.

     '16'
          Mark the glyph associated with this character as overlapping
          other instances of itself vertically.  Initially, the
          character '\[br]' has this property.

     '32'
          Mark the character as transparent for the purpose of
          end-of-sentence recognition.  In other words, an
          end-of-sentence character followed by any number of characters
          with this property is treated as the end of a sentence if
          followed by a newline or two spaces.  This is the same as
          having a zero space factor in TeX.  Initially, characters
          '"')]*\[dg]\[dd]\[rq]\[cq]' have this property.

     '64'
          Ignore hyphenation codes of the surrounding characters.  Use
          this in combination with values 2 and 4 (initially, no
          characters have this property).

          For example, if you need an automatic break point after the
          en-dash in numeric ranges like "3000-5000", insert

               .cflags 68 \[en]

          into your document.  Note, however, that this can lead to bad
          layout if done without thinking; in most situations, a better
          solution instead of changing the 'cflags' value is to insert
          '\:' right after the hyphen at the places that really need a
          break point.

     The remaining values were implemented for East Asian language
     support; those who use alphabetic scripts exclusively can disregard
     them.

     '128'
          Prohibit a line break before the character, but allow a line
          break after the character.  This works only in combination
          with flags 256 and 512 and has no effect otherwise.
          Initially, no characters have this property.

     '256'
          Prohibit a line break after the character, but allow a line
          break before the character.  This works only in combination
          with flags 128 and 512 and has no effect otherwise.
          Initially, no characters have this property.

     '512'
          Allow line break before or after the character.  This works
          only in combination with flags 128 and 256 and has no effect
          otherwise.  Initially, no characters have this property.

     In contrast to values 2 and 4, the values 128, 256, and 512 work
     pairwise.  If, for example, the left character has value 512, and
     the right character 128, no break will be automatically inserted
     between them.  If we use value 6 instead for the left character, a
     break after the character can't be suppressed since the neighboring
     character on the right doesn't get examined.

 -- Request: .char c [contents]
 -- Request: .fchar c [contents]
 -- Request: .fschar f c [contents]
 -- Request: .schar c [contents]
     Define a new character or glyph C to be CONTENTS, which can be
     empty.  More precisely, 'char' defines a 'groff' object (or
     redefines an existing one) that is accessed with the name C on
     input, and produces CONTENTS on output.  Every time glyph C needs
     to be printed, CONTENTS is processed in a temporary environment and
     the result is wrapped up into a single object.  Compatibility mode
     is turned off and the escape character is set to '\' while CONTENTS
     is processed.  Any emboldening, constant spacing, or track kerning
     is applied to this object rather than to individual glyphs in
     CONTENTS.

     An object defined by these requests can be used just like a normal
     glyph provided by the output device.  In particular, other
     characters can be translated to it with the 'tr' or 'trin'
     requests; it can be made the leader character with the 'lc'
     request; repeated patterns can be drawn with it using the '\l' and
     '\L' escape sequences; and words containing C can be hyphenated
     correctly if the 'hcode' request is used to give the object a
     hyphenation code.

     There is a special anti-recursion feature: use of the object within
     its own definition is handled like a normal character (not defined
     with 'char').

     The 'tr' and 'trin' requests take precedence if 'char' accesses the
     same symbol.

          .tr XY
          X
              => Y
          .char X Z
          X
              => Y
          .tr XX
          X
              => Z

     The 'fchar' request defines a fallback glyph: 'gtroff' only checks
     for glyphs defined with 'fchar' if it cannot find the glyph in the
     current font.  'gtroff' carries out this test before checking
     special fonts.

     'fschar' defines a fallback glyph for font F: 'gtroff' checks for
     glyphs defined with 'fschar' after the list of fonts declared as
     font-specific special fonts with the 'fspecial' request, but before
     the list of fonts declared as global special fonts with the
     'special' request.

     Finally, the 'schar' request defines a global fallback glyph:
     'gtroff' checks for glyphs defined with 'schar' after the list of
     fonts declared as global special fonts with the 'special' request,
     but before the already mounted special fonts.

     *Note Character Classes::.

 -- Request: .rchar c1 c2 ...
 -- Request: .rfschar f c1 c2 ...
     Remove the definitions of glyphs C1, C2, ..., undoing the effect of
     a 'char', 'fchar', or 'schar' request.

     Spaces and tabs are optional between CN arguments.

     The request 'rfschar' removes glyph definitions defined with
     'fschar' for font F.

   (1) This is "Normalization Form D" as documented in Unicode Standard
Annex #15 (<https://unicode.org/reports/tr15/>).

   (2) '\C' is actually a misnomer since it accesses an output glyph.

   (3) Output glyphs don't have such properties.  For GNU 'troff', a
glyph is a box numbered with an index into a font, a given height above
and depth below the baseline, and a width--nothing more.

5.19.5 Character Classes
------------------------

Classes are particularly useful for East Asian languages such as
Chinese, Japanese, and Korean, where the number of needed characters is
much larger than in European languages, and where large sets of
characters share the same properties.

 -- Request: .class name c1 c2 ...
     Define a character class (or simply "class") NAME comprising the
     characters C1, C2, and so on.

     A class thus defined can then be referred to in lieu of listing all
     the characters within it.  Currently, only the 'cflags' request can
     handle references to character classes.

     In the request's simplest form, each CN is a character (or special
     character).

          .class [quotes] ' \[aq] \[dq] \[oq] \[cq] \[lq] \[rq]

     Since class and glyph names share the same name space, it is
     recommended to start and end the class name with '[' and ']',
     respectively, to avoid collisions with existing character names
     defined by GNU 'troff' or the user (with 'char' and related
     requests).  This practice applies the presence of ']' in the class
     name to prevent the use of the special character escape form
     '\[...]', thus you must use the '\C' escape to access a class with
     such a name.

     You can also use a character range notation consisting of a start
     character followed by '-' and then an end character.  Internally,
     GNU 'troff' converts these two symbol names to Unicode code points
     (according to the 'groff' glyph list [GGL]), which then give the
     start and end value of the range.  If that fails, the class
     definition is skipped.

     Furthermore, classes can be nested.

          .class [prepunct] , : ; > }
          .class [prepunctx] \C'[prepunct]' \[u2013]-\[u2016]

     The class '[prepunctx]' thus contains the contents of the class
     '[prepunct]' as defined above (the set ', : ; > }'), and characters
     in the range between 'U+2013' and 'U+2016'.

     If you want to include '-' in a class, it must be the first
     character value in the argument list, otherwise it gets
     misinterpreted as part of the range syntax.

     It is not possible to use class names as end points of range
     definitions.

     A typical use of the 'class' request is to control line-breaking
     and hyphenation rules as defined by the 'cflags' request.  For
     example, to inhibit line breaks before the characters belonging to
     the 'prepunctx' class defined in the previous example, you can
     write the following.

          .cflags 2 \C'[prepunctx]'

     See the 'cflags' request in *note Using Symbols::, for more
     details.

5.19.6 Special Fonts
--------------------

Special fonts are those that 'gtroff' searches when it cannot find the
requested glyph in the current font.  The Symbol font is usually a
special font.

   'gtroff' provides the following two requests to add more special
fonts.  *Note Using Symbols::, for a detailed description of the glyph
searching mechanism in 'gtroff'.

   Usually, only non-TTY devices have special fonts.

 -- Request: .special [s1 s2 ...]
 -- Request: .fspecial f [s1 s2 ...]
     Use the 'special' request to define special fonts.  Initially, this
     list is empty.

     Use the 'fspecial' request to designate special fonts only when
     font F is active.  Initially, this list is empty.

     Previous calls to 'special' or 'fspecial' are overwritten; without
     arguments, the particular list of special fonts is set to empty.
     Special fonts are searched in the order they appear as arguments.

     All fonts that appear in a call to 'special' or 'fspecial' are
     loaded.

     *Note Using Symbols::, for the exact search order of glyphs.

5.19.7 Artificial Fonts
-----------------------

There are a number of requests and escape sequences for artificially
creating fonts.  These are largely vestiges of the days when output
devices did not have a wide variety of fonts, and when 'nroff' and
'troff' were separate programs.  Most of them are no longer necessary in
GNU 'troff'.  Nevertheless, they are supported.

 -- Escape sequence: \H'height'
 -- Escape sequence: \H'+height'
 -- Escape sequence: \H'-height'
 -- Register: \n[.height]
     Change (increment, decrement) the height of the current font, but
     not the width.  If HEIGHT is zero, restore the original height.
     Default scaling indicator is 'z'.

     The read-only register '.height' contains the font height as set by
     '\H'.

     Currently, only the '-Tps' and '-Tpdf' devices support this
     feature.

     '\H' doesn't produce an input token in GNU 'troff'.  As a
     consequence, it can be used in requests like 'mc' (which expects a
     single character as an argument) to change the font on the fly:

          .mc \H'+5z'x\H'0'

     In compatibility mode, 'gtroff' behaves differently: If an
     increment or decrement is used, it is always taken relative to the
     current type size and not relative to the previously selected font
     height.  Thus,

          .cp 1
          \H'+5'test \H'+5'test

     prints the word 'test' twice with the same font height (five points
     larger than the current font size).

 -- Escape sequence: \S'slant'
 -- Register: \n[.slant]
     Slant the current font by SLANT degrees.  Positive values slant to
     the right.  Only integer values are possible.

     The read-only register '.slant' contains the font slant as set by
     '\S'.

     Currently, only the '-Tps' and '-Tpdf' devices support this
     feature.

     '\S' doesn't produce an input token in GNU 'troff'.  As a
     consequence, it can be used in requests like 'mc' (which expects a
     single character as an argument) to change the font on the fly:

          .mc \S'20'x\S'0'

     This escape is incorrectly documented in the AT&T 'troff' manual;
     the slant is always set to an absolute value.

 -- Request: .ul [lines]
     The 'ul' request normally underlines subsequent lines if a TTY
     output device is used.  Otherwise, the lines are printed in italics
     (only the term 'underlined' is used in the following).  The single
     argument is the quantity of input lines to be underlined; with no
     argument, the next line is underlined.  If LINES is zero or
     negative, stop the effects of 'ul' (if it was active).  Requests
     and empty lines do not count for computing the number of underlined
     input lines, even if they produce some output like 'tl'.  Lines
     inserted by macros (e.g., invoked by a trap) do count.

     At the beginning of 'ul', the current font is stored and the
     underline font is activated.  Within the span of a 'ul' request, it
     is possible to change fonts, but after the last line affected by
     'ul' the saved font is restored.

     This number of lines still to be underlined is associated with the
     environment (*note Environments::).  The underline font can be
     changed with the 'uf' request.

     The 'ul' request does not underline spaces.

 -- Request: .cu [lines]
     The 'cu' request is similar to 'ul' but underlines spaces as well
     (if a TTY output device is used).

 -- Request: .uf font
     Set the underline font (globally) used by 'ul' and 'cu'.  By
     default, this is the font at position 2.  FONT can be either a
     non-negative font position or the name of a font.

 -- Request: .bd font [offset]
 -- Request: .bd font1 font2 [offset]
 -- Register: \n[.b]
     Artificially create a bold font by printing each glyph twice,
     slightly offset.

     Two syntax forms are available.

        * Imitate a bold font unconditionally.  The first argument
          specifies the font to embolden, and the second is the number
          of basic units, minus one, by which the two glyphs are offset.
          If the second argument is missing, emboldening is turned off.

          FONT can be either a non-negative font position or the name of
          a font.

          OFFSET is available in the '.b' read-only register if a
          special font is active; in the 'bd' request, its default unit
          is 'u'.

        * Imitate a bold form conditionally.  Embolden FONT1 by OFFSET
          only if font FONT2 is the current font.  This request can be
          issued repeatedly to set up different emboldening values for
          different current fonts.  If the second argument is missing,
          emboldening is turned off for this particular current font.

          This affects special fonts only (either set up with the
          'special' command in font files or with the 'fspecial'
          request).

 -- Request: .cs font [width [em-size]]
     Switch to and from "constant glyph space mode".  If activated, the
     width of every glyph is WIDTH/36 ems.  The em size is given
     absolutely by EM-SIZE; if this argument is missing, the em value is
     taken from the current font size (as set with the 'ps' request)
     when the font is effectively in use.  Without second and third
     argument, constant glyph space mode is deactivated.

     Default scaling indicator for EM-SIZE is 'z'; WIDTH is an integer.

5.19.8 Ligatures and Kerning
----------------------------

Ligatures are groups of characters that are run together, i.e, producing
a single glyph.  For example, the letters 'f' and 'i' can form a
ligature 'fi' as in the word 'file'.  This produces a cleaner look
(albeit subtle) to the printed output.  Usually, ligatures are not
available in fonts for TTY output devices.

   Most PostScript fonts support the fi and fl ligatures.  The C/A/T
typesetter that was the target of AT&T 'troff' also supported 'ff',
'ffi', and 'ffl' ligatures.  Advanced typesetters or 'expert' fonts may
include ligatures for 'ft' and 'ct', although GNU 'troff' does not
support these (yet).

   Only the current font is checked for ligatures and kerns; neither
special fonts nor entities defined with the 'char' request (and its
siblings) are taken into account.

 -- Request: .lg [flag]
 -- Register: \n[.lg]
     Switch the ligature mechanism on or off; if the parameter is
     non-zero or missing, ligatures are enabled, otherwise disabled.
     Default is on.  The current ligature mode can be found in the
     read-only register '.lg' (set to 1 or 2 if ligatures are enabled,
     0 otherwise).

     Setting the ligature mode to 2 enables the two-character ligatures
     (fi, fl, and ff) and disables the three-character ligatures (ffi
     and ffl).

   "Pairwise kerning" is another subtle typesetting mechanism that
modifies the distance between a glyph pair to improve readability.  In
most cases (but not always) the distance is decreased.  Typewriter-like
fonts and fonts for terminals where all glyphs have the same width don't
use kerning.

 -- Request: .kern [flag]
 -- Register: \n[.kern]
     Switch kerning on or off.  If the parameter is non-zero or missing,
     enable pairwise kerning, otherwise disable it.  The read-only
     register '.kern' is set to 1 if pairwise kerning is enabled,
     0 otherwise.

     If the font description file contains pairwise kerning information,
     glyphs from that font are kerned.  Kerning between two glyphs can
     be inhibited by placing '\&' between them: 'V\&A'.

     *Note Font Description File Format::.

   "Track kerning" expands or reduces the space between glyphs.  This
can be handy, for example, if you need to squeeze a long word onto a
single line or spread some text to fill a narrow column.  It must be
used with great care since it is usually considered bad typography if
the reader notices the effect.

 -- Request: .tkf f s1 n1 s2 n2
     Enable track kerning for font F.  If the current font is F the
     width of every glyph is increased by an amount between N1 and N2
     (N1, N2 can be negative); if the current type size is less than or
     equal to S1 the width is increased by N1; if it is greater than or
     equal to S2 the width is increased by N2; if the type size is
     greater than or equal to S1 and less than or equal to S2 the
     increase in width is a linear function of the type size.

     The default scaling indicator is 'z' for S1 and S2, 'p' for N1 and
     N2.

     The track kerning amount is added even to the rightmost glyph in a
     line; for large values it is thus recommended to increase the line
     length by the same amount to compensate.

   Sometimes, when typesetting letters of different fonts, more or less
space at such boundaries is needed.  Two escape sequences help with
this.

 -- Escape sequence: \/
     Apply an "italic correction": modify the spacing of the preceding
     glyph so that the distance between it and the following glyph is
     correct if the latter is of upright shape.  For example, if an
     italic 'f' is followed immediately by a roman right parenthesis,
     then in many fonts the top right portion of the 'f' overlaps the
     top left of the right parenthesis, which is ugly.  Use this escape
     sequence whenever an oblique glyph is immediately followed by an
     upright glyph without any intervening space.

 -- Escape sequence: \,
     Apply a "left italic correction": modify the spacing of the
     following glyph so that the distance between it and the preceding
     glyph is correct if the latter is of upright shape.  For example,
     if a roman left parenthesis is immediately followed by an
     italic 'f', then in many fonts the bottom left portion of the 'f'
     overlaps the bottom of the left parenthesis, which is ugly.  Use
     this escape sequence whenever an upright glyph is followed
     immediately by an oblique glyph without any intervening space.

5.19.9 Dummy Characters
-----------------------

As discussed in *note Requests and Macros::, the first character on an
input line is treated specially.  Further, formatting a glyph has many
consequences on formatter state (*note Environments::).  Occasionally,
we want to escape this context or embrace some of those consequences
without actually rendering a glyph to the output.

 -- Escape sequence: \&
     Interpolate a dummy character, which is constitutive of output but
     invisible.(1)  (*note Dummy Characters-Footnote-1::) Its presence
     alters the interpretation context of a subsequent input character,
     and enjoys several applications.

        * It prevents insertion of extra space after an end-of-sentence
          character (*note Sentences::).

               Test.
               Test.
                   => Test.  Test.
               Test.\&
               Test.
                   => Test. Test.

        * It prevents interpretation of a control character at the
          beginning of an input line.

               .Test
                   error-> warning: macro 'Test' not defined
               \&.Test
                   => .Test

        * It prevents kerning between two glyphs.

        * It permits the 'tr' request to remap a character to nothing
          (*note Character Translations::).

     The dummy character escape sequence sees use in macro definitions
     as a means of ensuring that arguments are treated as text even if
     they begin with spaces or control characters.

          .de HD \" typeset a simple bold heading
          .  sp
          .  ft B
          \&\\$1 \" exercise: remove the \&
          .  ft
          .  sp
          ..
          .HD .\|.\|.\|surprised?

   One way to think about the dummy character is to imagine placing the
symbol '&' in the input at a certain location; if doing so has all the
side effects on formatting that you desire except for sticking an ugly
ampersand in the midst of your text, the dummy character is what you
want in its place.

 -- Escape sequence: \)
     Interpolate a transparent dummy character--one that is transparent
     to end-of-sentence detection.  It behaves as '\&', except that '\&'
     is treated as letters and numerals normally are after '.', '?' and
     '!'; '\&' cancels end-of-sentence detection, and '\)' does not.

          .de Suffix-&
          .  nop \&\\$1
          ..
          .
          .de Suffix-)
          .  nop \)\\$1
          ..
          .
          Here's a sentence.\c
          .Suffix-& '
          Another one.\c
          .Suffix-) '
          And a third.
              => Here's a sentence.' Another one.'  And a third.

   (1) Opinions of this escape sequence's name abound.  "Zero-width
space" is a popular misnomer: 'roff' formatters do not treat it like a
space.  Ossanna called it a "non-printing, zero-width character", but
the character causes _output_ even though it does not "print".  If no
output line is pending, the dummy character starts one.  Contrast an
empty input document with one containing only '\&' (and a newline).  The
former produces no output; the latter, a blank page.

5.20 Manipulating Type Size and Vertical Spacing
================================================

These concepts were introduced in *note Page Geometry::.  The height of
a font's tallest glyph is one em, which is equal to the type size in
points.(1)  (*note Manipulating Type Size and Vertical
Spacing-Footnote-1::) A vertical spacing of less than 120% of the type
size can make a document hard to read.  Larger proportions can be useful
to spread the text for annotations or proofreader's marks.  By default,
GNU 'troff' uses 10 point type on 12 point spacing.  Typographers call
the difference between type size and vertical spacing "leading".(2)
(*note Manipulating Type Size and Vertical Spacing-Footnote-2::)

   (1) In text fonts, the tallest glyphs are typically parentheses.
Unfortunately, in many cases the actual dimensions of the glyphs in a
font do not closely match its declared type size!  For example, in the
standard PostScript font families, 10-point Times sets better with
9-point Helvetica and 11-point Courier than if all three were used at
10 points.

   (2) Pronounce "leading" to rhyme with "sledding"; it refers to the
use of lead metal (Latin: _plumbum_) in traditional typesetting.

5.20.1 Changing the Type Size
-----------------------------

 -- Request: .ps [size]
 -- Request: .ps +size
 -- Request: .ps -size
 -- Escape sequence: \ssize
 -- Register: \n[.s]
     Use the 'ps' request or the '\s' escape sequence to change
     (increase, decrease) the type size (in scaled points).  Specify
     SIZE as either an absolute type size, or as a relative change from
     the current size.  'ps' with no argument restores the previous
     size.  The 'ps' request's default scaling unit is 'z'.  If the
     requested size is non-positive, it is set to 1u.

     The read-only string-valued register '.s' interpolates the type
     size in points as a decimal fraction; it is associated with the
     environment (*note Environments::).  To obtain the type size in
     scaled points, interpolate the '.ps' register instead (*note Using
     Fractional Type Sizes::).

          snap, snap,
          .ps +2
          grin, grin,
          .ps +2
          wink, wink, \s+2nudge, nudge,\s+8 say no more!
          .ps 10

     The '\s' escape sequence supports a variety of syntax forms.

     '\sN'
          Set the type size to N points.  N must be a single digit.  If
          N is 0, restore the previous size.

     '\s+N'
     '\s-N'
          Increase or decrease the type size by N points.  N must be
          exactly one digit.

     '\s(NN'
          Set the type size to NN points.  NN must be exactly two
          digits.

     '\s+(NN'
     '\s-(NN'
     '\s(+NN'
     '\s(-NN'
          Increase or decrease the type size by NN points.  NN must be
          exactly two digits.

     *Note Using Fractional Type Sizes::, for further syntactical forms
     of the '\s' escape sequence that additionally accept decimal
     fractions.

   The '\s' escape sequence affects the environment immediately and
doesn't produce an input token.  Consequently, it can be used in
requests like 'mc', which expects a single character as an argument, to
change the type size on the fly.

     .mc \s[20]x\s[0]

 -- Request: .sizes s1 s2 ... sn [0]
     Some devices may permit only certain type sizes, in which case GNU
     'troff' rounds to the nearest permissible size.  The 'DESC' file
     normally specifies which type sizes are allowed by the device.

     Use the 'sizes' request to change the permissible sizes for the
     output device.  Arguments are in scaled points; *Note Using
     Fractional Type Sizes::.  Each can be a single type size (such as
     '12000'), or a range of sizes (such as '4000-72000').  You can
     optionally end the list with a zero.

5.20.2 Changing the Vertical Spacing
------------------------------------

 -- Request: .vs [space]
 -- Request: .vs +space
 -- Request: .vs -space
 -- Register: \n[.v]
     Change (increase, decrease) the vertical spacing by SPACE.  The
     default scaling unit is 'p'.  If 'vs' is called without an
     argument, the vertical spacing is reset to the previous value
     before the last call to 'vs'.  GNU 'gtroff' emits a warning in
     category 'range' if SPACE is negative; the vertical spacing is then
     set to the smallest possible positive value, the vertical motion
     quantum (as found in the '.V' register).

     '.vs 0' isn't saved in a diversion since it doesn't result in a
     vertical motion.  You must explicitly issue this request before
     calling the diversion.

     The read-only register '.v' contains the vertical spacing; it is
     associated with the environment (*note Environments::).

When a break occurs, GNU 'troff' performs the following procedure.

   * Move the drawing position vertically by the "extra pre-vertical
     line space", the minimum of all negative '\x' escape sequence
     arguments in the pending output line.

   * Move the drawing position vertically by the vertical line spacing.

   * Write out the pending output line.

   * Move the drawing position vertically by the "extra post-vertical
     line space", the maximum of all positive '\x' escape sequence
     arguments in the line that has just been output.

   * Move the drawing position vertically by the "post-vertical line
     spacing" (see below).

   Prefer 'vs' or 'pvs' over 'ls' to produce double-spaced documents.
'vs' and 'pvs' have finer granularity than 'ls'; moreover, some
preprocessors assume single spacing.  *Note Manipulating Spacing::,
regarding the '\x' escape sequence and the 'ls' request.

 -- Request: .pvs [space]
 -- Request: .pvs +space
 -- Request: .pvs -space
 -- Register: \n[.pvs]
     Change (increase, decrease) the post-vertical spacing by SPACE.
     The default scaling unit is 'p'.  If 'pvs' is called without an
     argument, the post-vertical spacing is reset to the previous value
     before the last call to 'pvs'.  GNU 'troff' emits a warning in
     category 'range' if SPACE is negative; the post-vertical spacing is
     then set to zero.

     The read-only register '.pvs' contains the post-vertical spacing;
     it is associated with the environment (*note Environments::).

5.20.3 Using Fractional Type Sizes
----------------------------------

AT&T 'troff' interpreted all type size measurements in points.  Combined
with integer arithmetic, this design choice made it impossible to
support, for instance, ten and a half-point type.  In GNU 'troff', an
output device can select a scaling factor that subdivides a point into
"scaled points".  A type size expressed in scaled points can thus
represent a non-integral type size.

   A "scaled point" is equal to 1/SIZESCALE points, where SIZESCALE is
specified in the device description file 'DESC', and defaults to 1.(1)
(*note Using Fractional Type Sizes-Footnote-1::) Requests and escape
sequences in GNU 'troff' interpret arguments that represent a type size
in scaled points, which the formatter multiplies by SIZESCALE and
converts to an integer.  Arguments treated in this way comprise those to
the escape sequences '\H' and '\s', to the request 'ps', the third
argument to the 'cs' request, and the second and fourth arguments to the
'tkf' request.  Scaled points may be specified explicitly with the 'z'
scaling unit.

   For example, if SIZESCALE is 1000, then a scaled point is one
thousandth of a point.  The request '.ps 10.5' is synonymous with '.ps
10.5z' and sets the type size to 10,500 scaled points, or 10.5 points.
Consequently, in GNU 'troff', the register '.s' can interpolate a
non-integral type size.

 -- Register: \n[.ps]
     This read-only register interpolates the type size in scaled
     points; it is associated with the environment (*note
     Environments::).

   It makes no sense to use the 'z' scaling unit in a numeric expression
whose default scaling unit is neither 'u' nor 'z', so GNU 'troff'
disallows this.  Similarly, it is nonsensical to use a scaling unit
other than 'z' or 'u' in a numeric expression whose default scaling unit
is 'z', and so GNU 'troff' disallows this as well.

   Another GNU 'troff' scaling unit, 's', multiplies by the number of
basic units in a scaled point.  Thus, '\n[.ps]s' is equal to '1m' by
definition.  Do not confuse the 's' and 'z' scaling units.

 -- Register: \n[.psr]
 -- Register: \n[.sr]
     Output devices may be limited in the type sizes they can employ.
     The '.s' and '.ps' registers represent the type size selected by
     the output driver as it understands a device's capability.  The
     last _requested_ type size is interpolated in scaled points by the
     read-only register '.psr' and in points as a decimal fraction by
     the read-only string-valued register '.sr'.  Both are associated
     with the environment (*note Environments::).

     For example, if a type size of 10.95 points is requested, and the
     nearest size permitted by a 'sizes' request (or a 'sizescale'
     directive in the device's 'DESC' file) is 11 points, the latter
     value is used by the output driver.

   The '\s' escape sequence offers the following syntax forms that work
with fractional type sizes and accept scaling units.  You may of course
give them integral arguments.  The delimited forms need not use the
neutral apostrophe; see *note Delimiters::.

'\s[N]'
'\s'N''
     Set the type size to N scaled points; N is a numeric expression
     with a default scaling unit of 'z'.

'\s[+N]'
'\s[-N]'
'\s+[N]'
'\s-[N]'
'\s'+N''
'\s'-N''
'\s+'N''
'\s-'N''
     Increase or decrease the type size by N scaled points; N is a
     numeric expression (which may start with a minus sign) with a
     default scaling unit of 'z'.

   (1) *Note Device and Font Description Files::.

5.21 Colors
===========

GNU 'troff' supports color output with a variety of color spaces and up
to 16 bits per channel.  Some devices, particularly terminals, may be
more limited.  When color support is enabled, two colors are current at
any given time: the "stroke color", with which glyphs, rules (lines),
and geometric objects like circles and polygons are drawn, and the "fill
color", which can be used to paint the interior of a closed geometric
figure.

 -- Request: .color [n]
 -- Register: \n[.color]
     If N is missing or non-zero, enable the output of color-related
     device-independent output commands (this is the default);
     otherwise, disable them.  This request sets a global flag; it does
     not produce an input token (*note Gtroff Internals::).

     The read-only register '.color' is 1 if colors are enabled,
     0 otherwise.

     Color can also be disabled with the '-c' command-line option.

 -- Request: .defcolor ident scheme color-component ...
     Define a color named IDENT.  SCHEME selects a color space and
     determines the quantity of required COLOR-COMPONENTs; it must be
     one of 'rgb' (three components), 'cmy' (three), 'cmyk' (four), or
     'gray' (one).  'grey' is accepted as a synonym of 'gray'.  The
     color components can be encoded as a single hexadecimal value
     starting with '#' or '##'.  The former indicates that each
     component is in the range 0-255 (0-FF), the latter the range
     0-65,535 (0-FFFF).

          .defcolor half gray #7f
          .defcolor pink rgb #FFC0CB
          .defcolor magenta rgb  ##ffff0000ffff

     Alternatively, each color component can be specified as a decimal
     fraction in the range 0-1, interpreted using a default scaling
     indicator of 'f', which multiplies its value by 65,536 (but clamps
     it at 65,535).

          .defcolor gray50 rgb 0.5 0.5 0.5
          .defcolor darkgreen rgb 0.1f 0.5f 0.2f

   Each output device has a color named 'default', which cannot be
redefined.  A device's default stroke and fill colors are not
necessarily the same.  For the 'dvi', 'html', 'pdf', 'ps', and 'xhtml'
output devices, GNU 'troff' automatically loads a macro file defining
many color names at startup.  By the same mechanism, the devices
supported by 'grotty' recognize the eight standard ISO 6429/EMCA-48
color names.(1)  (*note Colors-Footnote-1::)

 -- Request: .gcolor [color]
 -- Escape sequence: \mc
 -- Escape sequence: \m(co
 -- Escape sequence: \m[color]
 -- Register: \n[.m]
     Set the stroke color to COLOR.

          .gcolor red
          The next words
          .gcolor
          \m[red]are in red\m[]
          and these words are in the previous color.

     The escape sequence '\m[]' restores the previous stroke color, as
     does a 'gcolor' request without an argument.

     The name of the current stroke color is available in the read-only
     string-valued register '.m'; it is associated with the environment
     (*note Environments::).  It interpolates nothing when the stroke
     color is the default.

     '\m' doesn't produce an input token in GNU 'troff' (*note Gtroff
     Internals::).  It therefore can be used in requests like 'mc'
     (which expects a single character as an argument) to change the
     color on the fly:

          .mc \m[red]x\m[]

 -- Request: .fcolor [color]
 -- Escape sequence: \Mc
 -- Escape sequence: \M(co
 -- Escape sequence: \M[color]
 -- Register: \n[.M]
     Set fill color for objects drawn with '\D'...'' escape sequences.

     Create an ellipse with a red interior as follows.

          \M[red]\h'0.5i'\D'E 2i 1i'\M[]

     The escape sequence '\M[]' restores the previous fill color, as
     does an 'fcolor' request without an argument.

     The name of the current fill color is available in the read-only
     string-valued register '.M'; it is associated with the environment
     (*note Environments::).  It interpolates nothing when the fill
     color is the default.

     '\M' doesn't produce an input token in GNU 'troff'.

   (1) also known vulgarly as "ANSI colors"

5.22 Strings
============

GNU 'troff' supports strings primarily for user convenience.
Conventionally, if one would define a macro only to interpolate a small
amount of text, without invoking requests or calling any other macros,
one defines a string instead.  Only one string is predefined by the
language.

 -- String: \*[.T]
     Contains the name of the output device (for example, 'utf8' or
     'pdf').

   The 'ds' request creates a string with a specified name and contents
and the '\*' escape sequence dereferences its name, interpolating its
contents.  If the string named by the '\*' escape sequence does not
exist, it is defined as empty, nothing is interpolated, and a warning in
category 'mac' is emitted.  *Note Warnings::, for information about the
enablement and suppression of warnings.

 -- Request: .ds name [contents]
 -- Request: .ds1 name [contents]
 -- Escape sequence: \*n
 -- Escape sequence: \*(nm
 -- Escape sequence: \*[name [arg1 arg2 ...]]
     Define a string called NAME with contents CONTENTS.  If NAME
     already exists as an alias, the target of the alias is redefined;
     see 'als' and 'rm' below.  If 'ds' is called with only one
     argument, NAME is defined as an empty string.  Otherwise, GNU
     'troff' stores CONTENTS in copy mode.(1)  (*note
     Strings-Footnote-1::)

     The '\*' escape sequence interpolates a previously defined string
     variable NAME (one-character name N, two-character name NM).  The
     bracketed interpolation form accepts arguments that are handled as
     macro arguments are; recall *note Calling Macros::.  In contrast to
     macro calls, however, if a closing bracket ']' occurs in a string
     argument, that argument must be enclosed in double quotes.  '\*' is
     interpreted even in copy mode.  When defining strings, argument
     interpolations must be escaped if they are to reference parameters
     from the calling context; *Note Parameters::.

          .ds cite (\\$1, \\$2)
          Gray codes are explored in \*[cite Morgan 1998].
              => Gray codes are explored in (Morgan, 1998).

     *Caution:* Unlike other requests, the second argument to the 'ds'
     request consumes the remainder of the input line, including
     trailing spaces.  This means that comments on a line with such a
     request can introduce unwanted space into a string when they are
     set off from the material they annotate, as is conventional.

          .ds H2O H\v'+.3m'\s'-2'2\v'-.3m'\s0O \" water

     Instead, place the comment on another line or put the comment
     escape sequence immediately adjacent to the last character of the
     string.

          .ds H2O H\v'+.3m'\s'-2'2\v'-.3m'\s0O\" water

     Ending string definitions (and appendments) with a comment, even an
     empty one, prevents unwanted space from creeping into them during
     source document maintenance.

          .ds author Alice Pleasance Liddell\"
          .ds empty \" might be appended to later with .as

     An initial neutral double quote '"' in CONTENTS is stripped to
     allow embedding of leading spaces.  Any other '"' is interpreted
     literally, but it is wise to use the special character escape
     sequence '\[dq]' instead if the string might be interpolated as
     part of a macro argument; see *note Calling Macros::.

          .ds salutation "         Yours in a white wine sauce,\"
          .ds c-var-defn "  char mydate[]=\[dq]2020-07-29\[dq];\"

     Strings are not limited to a single input line of text.  '\<RET>'
     works just as it does elsewhere.  The resulting string is stored
     _without_ the newlines.  Care is therefore required when
     interpolating strings while filling is disabled.

          .ds foo This string contains \
          text on multiple lines \
          of input.

     It is not possible to embed a newline in a string that will be
     interpreted as such when the string is interpolated.  To achieve
     that effect, use '\*' to interpolate a macro instead; see *note
     Punning Names::.

     Because strings are similar to macros, they too can be defined so
     as to suppress AT&T 'troff' compatibility mode when used; see *note
     Writing Macros:: and *note Compatibility Mode::.  The 'ds1' request
     defines a string such that compatibility mode is off when the
     string is later interpolated.  To be more precise, a "compatibility
     save" input token is inserted at the beginning of the string, and a
     "compatibility restore" input token at the end.

          .nr xxx 12345
          .ds aa The value of xxx is \\n[xxx].
          .ds1 bb The value of xxx is \\n[xxx].
          .
          .cp 1
          .
          \*(aa
              error-> warning: register '[' not defined
              => The value of xxx is 0xxx].
          \*(bb
              => The value of xxx is 12345.

 -- Request: .as name [contents]
 -- Request: .as1 name [contents]
     The 'as' request is similar to 'ds' but appends CONTENTS to the
     string stored as NAME instead of redefining it.  If NAME doesn't
     exist yet, it is created.  If 'as' is called with only one
     argument, no operation is performed (beyond dereferencing the
     string).

          .as salutation " with shallots, onions and garlic,\"

     The 'as1' request is similar to 'as', but compatibility mode is
     switched off when the appended portion of the string is later
     interpolated.  To be more precise, a "compatibility save" input
     token is inserted at the beginning of the appended string, and a
     "compatibility restore" input token at the end.

   Several requests exist to perform rudimentary string operations.
Strings can be queried ('length') and modified ('chop', 'substring',
'stringup', 'stringdown'), and their names can be manipulated through
renaming, removal, and aliasing ('rn', 'rm', 'als').

 -- Request: .length reg anything
     Compute the number of characters of ANYTHING and store the count in
     the register REG.  If REG doesn't exist, it is created.  ANYTHING
     is read in copy mode.

          .ds xxx abcd\h'3i'efgh
          .length yyy \*[xxx]
          \n[yyy]
              => 14

 -- Request: .chop object
     Remove the last character from the macro, string, or diversion
     named OBJECT.  This is useful for removing the newline from the end
     of a diversion that is to be interpolated as a string.  This
     request can be used repeatedly on the same OBJECT; see *note Gtroff
     Internals::, for details on nodes inserted additionally by GNU
     'troff'.

 -- Request: .substring str start [end]
     Replace the string named STR with its substring bounded by the
     indices START and END, inclusively.  The first character in the
     string has index 0.  If END is omitted, it is implicitly set to the
     largest valid value (the string length minus one).  Negative
     indices count backward from the end of the string: the last
     character has index -1, the character before the last has index -2,
     and so on.

          .ds xxx abcdefgh
          .substring xxx 1 -4
          \*[xxx]
              => bcde
          .substring xxx 2
          \*[xxx]
              => de

 -- Request: .stringdown str
 -- Request: .stringup str
     Alter the string named STR by replacing each of its bytes with its
     lowercase ('stringdown') or uppercase ('stringup') version (if one
     exists).  Special characters in the string will often transform in
     the expected way due to the regular naming convention for accented
     characters.  When they do not, use substrings and/or catenation.

          .ds resume R\['e]sum\['e]
          \*[resume]
          .stringdown resume
          \*[resume]
          .stringup resume
          \*[resume]
              => Rsum rsum RSUM

   (In practice, we would end the 'ds' request with a comment escape
'\"' to prevent space from creeping into the definition during source
document maintenance.)

 -- Request: .rn old new
     Rename the request, macro, diversion, or string OLD to NEW.

 -- Request: .rm name
     Remove the request, macro, diversion, or string NAME.  GNU 'troff'
     treats subsequent invocations as if the name had never been
     defined.

 -- Request: .als new old
     Create an alias NEW for the existing request, string, macro, or
     diversion object named OLD, causing the names to refer to the same
     stored object.  If OLD is undefined, a warning in category 'mac' is
     produced, and the request is ignored.  *Note Warnings::, for
     information about the enablement and suppression of warnings.

     To understand how the 'als' request works, consider two different
     storage pools: one for objects (macros, strings, etc.), and another
     for names.  As soon as an object is defined, GNU 'troff' adds it to
     the object pool, adds its name to the name pool, and creates a link
     between them.  When 'als' creates an alias, it adds a new name to
     the name pool that gets linked to the same object as the old name.

     Now consider this example.

          .de foo
          ..
          .
          .als bar foo
          .
          .de bar
          .  foo
          ..
          .
          .bar
              error-> input stack limit exceeded (probable infinite
              error-> loop)

     In the above, 'bar' remains an _alias_--another name for--the
     object referred to by 'foo', which the second 'de' request
     replaces.  Alternatively, imagine that the 'de' request
     _dereferences_ its argument before replacing it.  Either way, the
     result of calling 'bar' is a recursive loop that finally leads to
     an error.  *Note Writing Macros::.

     To remove an alias, call 'rm' on its name.  The object itself is
     not destroyed until it has no more names.

     When a request, macro, string, or diversion is aliased,
     redefinitions and appendments "write through" alias names.  To
     replace an alias with a separately defined object, you must use the
     'rm' request on its name first.

   (1) *Note Copy Mode::.

5.23 Conditionals and Loops
===========================

'groff' has 'if' and 'while' control structures like other languages.
However, the syntax for grouping multiple input lines in the branches or
bodies of these structures is unusual.

5.23.1 Operators in Conditionals
--------------------------------

In 'if', 'ie', and 'while' requests, in addition to the numeric
expressions described in *note Numeric Expressions::, several Boolean
operators are available; the members of this expanded class are termed
"conditional expressions".

'c GLYPH'
     True if GLYPH is available, where GLYPH is a Unicode basic Latin
     character, a GNU 'troff' special character '\(XX' or '\[XXX]',
     '\N'XXX'', or has been defined by any of the 'char', 'fchar',
     'fschar', or 'schar' requests.

'd NAME'
     True if a string, macro, diversion, or request called NAME exists.

'e'
     True if the current page is even-numbered.

'F FONT'
     True if FONT exists.  FONT is handled as if it were opened with the
     'ft' request (that is, font translation and styles are applied),
     without actually mounting it.

'm COLOR'
     True if COLOR is defined.

'n'
     True if the document is being processed in 'nroff' mode.  *Note
     troff and nroff Modes::.

'o'
     True if the current page is odd-numbered.

'r REGISTER'
     True if REGISTER exists.

'S STYLE'
     True if STYLE is available for the current font family.  Font
     translation is applied.

't'
     True if the document is being processed in 'troff' mode.  *Note
     troff and nroff Modes::.

'v'
     Always false.  This condition is recognized only for compatibility
     with certain other 'troff' implementations.(1)  (*note Operators in
     Conditionals-Footnote-1::)

   If the first argument to an 'if', 'ie', or 'while' request begins
with a non-alphanumeric character apart from '!' (see below); it
performs an output comparison test.  (2)  (*note Operators in
Conditionals-Footnote-2::)

''XXX'YYY''
     True if formatting the comparands XXX and YYY produces the same
     output commands.  The delimiter need not be a neutral apostrophe:
     the output comparison operator accepts the same delimiters as most
     escape sequences; see *note Delimiters::.  This "output comparison
     operator" formats XXX and YYY in separate environments; after the
     comparison, the resulting data are discarded.

          .ie "|"\fR|\fP" true
          .el false
              => true

     The resulting glyph properties, including font family, style, size,
     and slant, must match, but not necessarily the requests and/or
     escape sequences used to obtain them.  In the previous example, '|'
     and '\fR|\fP' result in '|' glyphs in the same typefaces at the
     same positions, so the comparands are equal.  If '.ft I' had been
     added before the '.ie', they would differ: the first '|' would
     produce an italic '|', not a roman one.  Motions must match in
     orientation and magnitude to within the applicable horizontal and
     vertical motion quanta of the device, after rounding.  '.if
     "\u\d"\v'0'"' is false even though both comparands result in zero
     net motion, because motions are not interpreted or optimized but
     sent as-is to the output.(3)  (*note Operators in
     Conditionals-Footnote-3::) On the other hand, '.if "\d"\v'0.5m'"'
     is true, because '\d' is defined as a downward motion of one-half
     em.(4)  (*note Operators in Conditionals-Footnote-4::)

     Surround the comparands with '\?' to avoid formatting them; this
     causes them to be compared character by character, as with string
     comparisons in other programming languages.

          .ie "\?|\?"\?\fR|\fP\?" true
          .el false
              => false

     Since comparands protected with '\?' are read in copy mode (*note
     Copy Mode::), they need not even be valid 'groff' syntax.  The
     escape character is still lexically recognized, however, and
     consumes the next character.

          .ds a \[
          .ds b \[
          .if '\?\*a\?'\?\*b\?' a and b equivalent
          .if '\?\\?'\?\\?' backslashes equivalent
              => a and b equivalent

   The above operators can't be combined with most others, but a leading
'!', not followed immediately by spaces or tabs, complements an
expression.

     .nr x 1
     .ie !r x register x is not defined
     .el      register x is defined
         => register x is defined

   Spaces and tabs are optional immediately after the 'c', 'd', 'F',
'm', 'r', and 'S' operators, but right after '!', they end the predicate
and the conditional evaluates true.(5)  (*note Operators in
Conditionals-Footnote-5::)

     .nr x 1
     .ie ! r x register x is not defined
     .el       register x is defined
         => r x register x is not defined

The unexpected 'r x' in the output is a clue that our conditional was
not interpreted as we planned, but matters may not always be so obvious.

   (1) This refers to 'vtroff', a translator that would convert the
C/A/T output from early-vintage AT&T 'troff' to a form suitable for
Versatec and Benson-Varian plotters.

   (2) Strictly, letters not otherwise recognized _are_ treated as
output comparison delimiters.  For portability, it is wise to avoid
using letters not in the list above; for example, Plan 9 'troff' uses
'h' to test a mode it calls 'htmlroff', and GNU 'troff' may provide
additional operators in the future.

   (3) Because formatting of the comparands takes place in a dummy
environment, vertical motions within them cannot spring traps.

   (4) All of this is to say that the lists of output nodes created by
formatting XXX and YYY must be identical.  *Note Gtroff Internals::.

   (5) This bizarre behavior maintains compatibility with AT&T 'troff'.

5.23.2 if-then
--------------

 -- Request: .if cond-expr anything
     Evaluate the conditional expression COND-EXPR, and if it evaluates
     true (or to a positive value), interpret the remainder of the line
     ANYTHING as if it were an input line.  Recall from *note Invoking
     Requests:: that any quantity of spaces between arguments to
     requests serves only to separate them; leading spaces in ANYTHING
     are thus not seen.  ANYTHING effectively _cannot_ be omitted; if
     COND-EXPR is true and ANYTHING is empty, the newline at the end of
     the control line is interpreted as a blank input line (and
     therefore a blank text line).

          super\c
          tanker
          .nr force-word-break 1
          super\c
          .if ((\n[force-word-break] = 1) & \n[.int])
          tanker
              => supertanker super tanker

 -- Request: .nop anything
     Interpret ANYTHING as if it were an input line.  This is similar to
     '.if 1'.  'nop' is not really "no operation"; its argument _is_
     processed--unconditionally.  It can be used to cause text lines to
     share indentation with surrounding control lines.

          .als real-MAC MAC
          .de wrapped-MAC
          .  tm MAC: called with arguments \\$@
          .  nop \\*[real-MAC]\\
          ..
          .als MAC wrapped-MAC
          \# Later...
          .als MAC real-MAC

     In the above, we've used aliasing, 'nop', and the interpolation of
     a macro as a string to interpose a wrapper around the macro 'MAC'
     (perhaps to debug it).

5.23.3 if-else
--------------

 -- Request: .ie cond-expr anything
 -- Request: .el anything
     Use the 'ie' and 'el' requests to write an if-then-else.  The first
     request is the "if" part and the latter is the "else" part.
     Unusually among programming languages, any number of
     non-conditional requests may be interposed between the 'ie' branch
     and the 'el' branch.

          .nr a 0
          .ie \na a is nonzero.
          .nr a +1
          .el a was not positive but is now \na.
              => a was not positive but is now 1.

     Another way in which 'el' is an ordinary request is that it does
     not lexically "bind" more tightly to its 'ie' counterpart than it
     does to any other request.  This fact can surprise C programmers.

          .nr a 1
          .nr z 0
          .ie \nz \
          .  ie \na a is true
          .  el     a is false
          .el z is false
              error-> warning: unbalanced 'el' request
              => a is false

     To conveniently nest conditionals, keep reading.

5.23.4 Conditional Blocks
-------------------------

 -- Escape sequence: \{
 -- Escape sequence: \}
     It is frequently desirable for a control structure to govern more
     than one request, macro call, text line, or a combination of the
     foregoing.  The opening and closing brace escape sequences '\{' and
     '\}' perform such grouping; such "conditional blocks" can be
     nested.  Brace escape sequences outside of control structures have
     no meaning and produce no output.

     '\{' should appear (after optional spaces and tabs) immediately
     subsequent to the request's conditional expression.  '\}' should
     appear on a line with other occurrences of itself as necessary to
     match '\{' sequences.  It can be preceded by a control character,
     spaces, and tabs.  Input after any quantity of '\}' sequences on
     the same line is processed only if all of the preceding conditions
     to which they correspond are true.  Furthermore, a '\}' closing the
     body of a 'while' request (discussed below) must be the last such
     escape sequence on an input line.

          A
          .if 0 \{ B
          C
          D
          \}E
          F
              => A F

          N
          .if 1 \{ O
          .  if 0 \{ P
          Q
          R\} S\} T
          U
              => N O U

     The above behavior may challenge the intuition; it was implemented
     to retain compatibility with AT&T 'troff'.  For clarity, it is
     idiomatic to end input lines with '\{', followed by '\<RET>' if
     desired to prevent the newline from being interpreted as a blank
     text line, and to precede '\}' on an input line with nothing more
     than a control character, spaces, tabs, and other instances of
     itself.

          .de DEBUG
          debug =
          .ie \\$1 \{\
          ON,
          development
          \}
          .el \{\
          OFF,
          production
          \}
          version
          ..
          .DEBUG 0
          .br
          .DEBUG 1

     Try omitting the '\<RET>'s from the foregoing example and see how
     the output changes.  Remember that, as noted above, after a true
     conditional (or after the 'el' request if its counterpart 'ie'
     condition was false) the remainder of the input line is interpreted
     _as if it were on an input line by itself_.

   We can use 'ie', 'el', and conditional blocks to simulate the
multi-way "switch" or "case" control structures of other languages.  The
following example is adapted from the 'groff' 'man' package.
Indentation is used to clarify the logic.

     .\" Simulate switch/case in roff.
     .      ie '\\$2'1' .ds title General Commands\"
     .el \{.ie '\\$2'2' .ds title System Calls\"
     .el \{.ie '\\$2'3' .ds title Library Functions\"
     .el \{.ie '\\$2'4' .ds title Kernel Interfaces\"
     .el \{.ie '\\$2'5' .ds title File Formats\"
     .el \{.ie '\\$2'6' .ds title Games\"
     .el \{.ie '\\$2'7' .ds title Miscellaneous Information\"
     .el \{.ie '\\$2'8' .ds title System Management\"
     .el \{.ie '\\$2'9' .ds title Kernel Development\"
     .el                .ds title \" empty
     .\}\}\}\}\}\}\}\}

5.23.5 while
------------

'groff' provides a looping construct: the 'while' request.  Its syntax
matches the 'if' request.

 -- Request: .while cond-expr anything
     Evaluate the conditional expression COND-EXPR, and repeatedly
     execute ANYTHING unless and until COND-EXPR evaluates false.
     ANYTHING, which is often a conditional block, is referred to as the
     'while' request's "body".

          .nr a 0 1
          .while (\na < 9) \{\
          \n+a,
          .\}
          \n+a
              => 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

     GNU 'troff' treats the body of a 'while' request similarly to that
     of a 'de' request (albeit one not read in copy mode(1) (*note
     while-Footnote-1::)), but stores it under an internal name and
     deletes it when the loop finishes.  The operation of a macro
     containing a 'while' request can slow significantly if the 'while'
     body is large.  Each time the macro is executed, the 'while' body
     is parsed and stored again.

          .de xxx
          .  nr num 10
          .  while (\\n[num] > 0) \{\
          .    \" many lines of code
          .    nr num -1
          .  \}
          ..

     An often better solution--and one that is more portable, since AT&T
     'troff' lacked the 'while' request--is to instead write a recursive
     macro.  It will be parsed only once.(2)  (*note while-Footnote-2::)

          .de yyy
          .  if (\\n[num] > 0) \{\
          .    \" many lines of code
          .    nr num -1
          .    yyy
          .  \}
          ..
          .
          .de xxx
          .  nr num 10
          .  yyy
          ..

     To prevent infinite loops, the default number of available
     recursion levels is 1,000 or somewhat less.(3)  (*note
     while-Footnote-3::) You can disable this protective measure, or
     raise the limit, by setting the 'slimit' register.  *Note
     Debugging::.

     As noted above, if a 'while' body begins with a conditional block,
     its closing brace must end an input line.

          .if 1 \{\
          .  nr a 0 1
          .  while (\n[a] < 10) \{\
          .    nop \n+[a]
          .\}\}
              error-> unbalanced brace escape sequences

 -- Request: .break
     Exit a 'while' loop.  Do not confuse this request with a
     typographical break or the 'br' request.

 -- Request: .continue
     Skip the remainder of a 'while' loop's body, immediately starting
     the next iteration.

   (1) *Note Copy Mode::.

   (2) unless you redefine it

   (3) "somewhat less" because things other than macro calls can be on
the input stack

5.24 Writing Macros
===================

A "macro" is a stored collection of text and control lines that can be
interpolated multiple times.  Use macros to define common operations.
Macros are called in the same way that requests are invoked.  While
requests exist for the purpose of creating macros, simply calling an
undefined macro, or interpolating it as a string, will cause it to be
defined as empty.  *Note Identifiers::.

 -- Request: .de name [end]
     Define a macro NAME, replacing the definition of any existing
     request, macro, string, or diversion called NAME.  If NAME already
     exists as an alias, the target of the alias is redefined; recall
     *note Strings::.  GNU 'troff' enters copy mode,(1) (*note Writing
     Macros-Footnote-1::) storing subsequent input lines as the macro
     definition.  If the optional second argument is not specified, the
     definition ends with the control line '..' (two dots).
     Alternatively, END identifies a macro whose call syntax at the
     start of a control line ends the definition of NAME; END is then
     called normally.  A macro definition must end in the same
     conditional block (if any) in which it began (*note Conditional
     Blocks::).  Spaces or tabs are permitted after the control
     character in the line containing this ending token (either '.' or
     'END'), but a tab immediately after the token prevents its
     recognition as the end of a macro definition.  The macro END can be
     called with arguments.(2)  (*note Writing Macros-Footnote-2::)

     Here is a small example macro called 'P' that causes a break and
     inserts some vertical space.  It could be used to separate
     paragraphs.

          .de P
          .  br
          .  sp .8v
          ..

     We can define one macro within another.  Attempting to nest '..'
     navely will end the outer definition because the inner definition
     isn't interpreted as such until the outer macro is later
     interpolated.  We can use an end macro instead.  Each level of
     nesting should use a unique end macro.

     An end macro need not be defined until it is called.  This fact
     enables a nested macro definition to begin inside one macro and end
     inside another.  Consider the following example.(3)  (*note Writing
     Macros-Footnote-3::)

          .de m1
          .  de m2 m3
          you
          ..
          .de m3
          Hello,
          Joe.
          ..
          .de m4
          do
          ..
          .m1
          know?
          .  m3
          What
          .m4
          .m2
              => Hello, Joe.  What do you know?

     A nested macro definition _can_ be terminated with '..' and nested
     macros _can_ reuse end macros, but these control lines must be
     escaped multiple times for each level of nesting.  The necessity of
     this escaping and the utility of nested macro definitions will
     become clearer when we employ macro parameters and consider the
     behavior of copy mode in detail.

   'de' defines a macro that inherits the compatibility mode enablement
status of its context (*note Implementation Differences::).  Often it is
desirable to make a macro that uses 'groff' features callable from
contexts where compatibility mode is on; for instance, when writing
extensions to a historical macro package.  To achieve this,
compatibility mode needs to be switched off while such a macro is
interpreted--without disturbing that state when it is finished.

 -- Request: .de1 name [end]
     The 'de1' request defines a macro to be interpreted with
     compatibility mode disabled.  When NAME is called, compatibility
     mode enablement status is saved; it is restored when the call
     completes.  Observe the extra backlash before the interpolation of
     register 'xxx'; we'll explore this subject in *note Copy Mode::.

          .nr xxx 12345
          .de aa
          The value of xxx is \\n[xxx].
          .  br
          ..
          .de1 bb
          The value of xxx is \\n[xxx].
          ..
          .cp 1
          .aa
              error-> warning: register '[' not defined
              => The value of xxx is 0xxx].
          .bb
              => The value of xxx is 12345.

 -- Request: .dei name [end]
 -- Request: .dei1 name [end]
     The 'dei' request defines a macro with its name and end macro
     indirected through strings.  That is, it interpolates strings named
     NAME and END before performing the definition.

     The following examples are equivalent.

          .ds xx aa
          .ds yy bb
          .dei xx yy

          .de aa bb

     The 'dei1' request bears the same relationship to 'dei' as 'de1'
     does to 'de'; it temporarily turns compatibility mode off when NAME
     is called.

 -- Request: .am name [end]
 -- Request: .am1 name [end]
 -- Request: .ami name [end]
 -- Request: .ami1 name [end]
     'am' appends subsequent input lines to macro NAME, extending its
     definition, and otherwise working as 'de' does.

     To make the previously defined 'P' macro set indented instead of
     block paragraphs, add the necessary code to the existing macro.

          .am P
          .ti +5n
          ..

     The other requests are analogous to their 'de' counterparts.  The
     'am1' request turns off compatibility mode during interpretation of
     the appendment.  The 'ami' request appends indirectly, meaning that
     strings NAME and END are interpolated with the resulting names used
     before appending.  The 'ami1' request is similar to 'ami',
     disabling compatibility mode during interpretation of the appended
     lines.

   Using 'trace.tmac', you can trace calls to 'de', 'de1', 'am', and
'am1'.  You can also use the 'backtrace' request at any point desired to
troubleshoot tricky spots (*note Debugging::).

   *Note Strings::, for the 'als', 'rm', and 'rn' requests to create an
alias of, remove, and rename a macro, respectively.

   Macro identifiers share their name space with requests, strings, and
diversions; see *note Identifiers::.  The 'am', 'as', 'da', 'de', 'di',
and 'ds' requests (together with their variants) create a new object
only if the name of the macro, diversion, or string is currently
undefined or if it is defined as a request; normally, they modify the
value of an existing object.  *Note the description of the 'als'
request: als, for pitfalls when redefining a macro that is aliased.

 -- Request: .return [anything]
     Exit a macro, immediately returning to the caller.  If called with
     an argument ANYTHING, exit twice--the current macro and the macro
     one level higher.  This is used to define a wrapper macro for
     'return' in 'trace.tmac'.

   (1) *Note Copy Mode::.

   (2) While it is possible to define and call a macro '.', you can't
use it as an end macro: during a macro definition, '..' is never handled
as calling '.', even if '.de NAME .' explicitly precedes it.

   (3) Its structure is adapted from, and isomorphic to, part of a
solution by Tadziu Hoffman to the problem of reflowing text multiple
times to find an optimal configuration for it.
<https://lists.gnu.org/archive/html/groff/2008-12/msg00006.html>

5.24.1 Parameters
-----------------

Macro calls and string interpolations optionally accept a list of
arguments; recall *note Calling Macros::.  At the time such an
interpolation takes place, these "parameters" can be examined using a
register and a variety of escape sequences starting with '\$'.  All such
escape sequences are interpreted even in copy mode, a fact we shall
motivate and explain below (*note Copy Mode::).

 -- Register: \n[.$]
     The count of parameters available to a macro or string is kept in
     this read-only register.  The 'shift' request can change its value.

   Any individual parameter can be accessed by its position in the list
of arguments to the macro call, numbered from left to right starting at
1, with one of the following escape sequences.

 -- Escape sequence: \$n
 -- Escape sequence: \$(nn
 -- Escape sequence: \$[nnn]
     Interpolate the Nth, NNth, or NNNth parameter.  The first form
     expects only a single digit (1<=N<=9)), the second two digits
     (01<=NN<=99)), and the third any positive integer NNN.  Macros and
     strings accept an unlimited number of parameters.

 -- Request: .shift [n]
     Shift the parameters N places (1 by default).  This is a "left
     shift": what was parameter I becomes parameter I-N.  The parameters
     formerly in positions 1 to N are no longer available.  Shifting by
     a nonpositive amount performs no operation.  The register '.$' is
     adjusted accordingly.

   In practice, parameter interpolations are usually seen prefixed with
an extra escape character.  This is because the '\$' family of escape
sequences is interpreted even in copy mode.(1)  (*note
Parameters-Footnote-1::)

 -- Escape sequence: \$*
 -- Escape sequence: \$@
 -- Escape sequence: \$^
     In some cases it is convenient to interpolate all of the parameters
     at once (to pass them to a request, for instance).  The '\$*'
     escape concatenates the parameters, separating them with spaces.
     '\$@' is similar, concatenating the parameters, surrounding each
     with double quotes and separating them with spaces.  If not in
     compatibility mode, the interpolation depth of double quotes is
     preserved (*note Calling Macros::).  '\$^' interpolates all
     parameters as if they were arguments to the 'ds' request.

          .de foo
          . tm $1='\\$1'
          . tm $2='\\$2'
          . tm $*='\\$*'
          . tm $@='\\$@'
          . tm $^='\\$^'
          ..
          .foo " This is a "test"
              error-> $1=' This is a '
              error-> $2='test"'
              error-> $*=' This is a  test"'
              error-> $@='" This is a " "test""'
              error-> $^='" This is a "test"'

     '\$*' is useful when writing a macro that doesn't need to
     distinguish its arguments, or even to not interpret them; examples
     include macros that produce diagnostic messages by wrapping the
     'tm' or 'ab' requests.  Use '\$@' when writing a macro that may
     need to shift its parameters and/or wrap a macro or request that
     finds the count significant.  If in doubt, prefer '\$@' to '\$*'.
     An application of '\$^' is seen in 'trace.tmac', which redefines
     some requests and macros for debugging purposes.

 -- Escape sequence: \$0
     Interpolate the name by which the macro being interpreted was
     called.  The 'als' request can cause a macro to have more than one
     name.  Applying string interpolation to a macro does not change
     this name.

          .de foo
          .  tm \\$0
          ..
          .als bar foo
          .
          .de aaa
          .  foo
          ..
          .de bbb
          .  bar
          ..
          .de ccc
          \\*[foo]\\
          ..
          .de ddd
          \\*[bar]\\
          ..
          .
          .aaa
              error-> foo
          .bbb
              error-> bar
          .ccc
              error-> ccc
          .ddd
              error-> ddd

   (1) If they were not, parameter interpolations would be similar to
command-line parameters--fixed for the entire duration of a 'roff'
program's run.  The advantage of interpolating '\$' escape sequences
even in copy mode is that they can interpolate different contents from
one call to the next, like function parameters in a procedural language.
The additional escape character is the price of this power.

5.24.2 Copy Mode
----------------

When GNU 'troff' processes certain requests, most importantly those
which define or append to a macro or string, it does so in "copy mode":
it copies the characters of the definition into a dedicated storage
region, interpolating the escape sequences '\n', '\g', '\$', '\*', '\V',
and '\?' normally; interpreting '\<RET>' immediately; discarding
comments '\"' and '\#'; interpolating the current leader, escape, or tab
character with '\a', '\e', and '\t', respectively; and storing all other
escape sequences in an encoded form.

   The complement of copy mode--a 'roff' formatter's behavior when not
defining or appending to a macro, string, or diversion--where all macros
are interpolated, requests invoked, and valid escape sequences processed
immediately upon recognition, can be termed "interpretation mode".

 -- Escape sequence: \\
     The escape character, '\' by default, can escape itself.  This
     enables you to control whether a given '\n', '\g', '\$', '\*',
     '\V', or '\?' escape sequence is interpreted at the time the macro
     containing it is defined, or later when the macro is called.(1)
     (*note Copy Mode-Footnote-1::)

          .nr x 20
          .de y
          .nr x 10
          \&\nx
          \&\\nx
          ..
          .y
              => 20 10

     You can think of '\\' as a "delayed" backslash; it is the escape
     character followed by a backslash from which the escape character
     has removed its special meaning.  Consequently, '\\' is not an
     escape sequence in the usual sense.  In any escape sequence '\X'
     that GNU 'troff' does not recognize, the escape character is
     ignored and X is output.  An unrecognized escape sequence causes a
     warning in category 'escape', with two exceptions--'\\' is the
     first.

 -- Escape sequence: \.
     '\.' escapes the control character.  It is similar to '\\' in that
     it isn't a true escape sequence.  It is used to permit nested macro
     definitions to end without a named macro call to conclude them.
     Without a syntax for escaping the control character, this would not
     be possible.

          .de m1
          foo
          .
          .  de m2
          bar
          \\..
          .
          ..
          .m1
          .m2
              => foo bar

     The first backslash is consumed while the macro is read, and the
     second is interpreted when macro 'm1' is called.

   'roff' documents should not use the '\\' or '\.' character sequences
outside of copy mode; they serve only to obfuscate the input.  Use '\e'
to represent the escape character, '\[rs]' to obtain a backslash glyph,
and '\&' before '.' and ''' where GNU 'troff' expects them as control
characters if you mean to use them literally (recall *note Requests and
Macros::).

   Macro definitions can be nested to arbitrary depth.  The mechanics of
parsing the escape character have significant consequences for this
practice.

     .de M1
     \\$1
     .  de M2
     \\\\$1
     .    de M3
     \\\\\\\\$1
     \\\\..
     .    M3 hand.
     \\..
     .  M2 of
     ..
     This understeer is getting
     .M1 out
         => This understeer is getting out of hand.

   Each escape character is interpreted twice--once in copy mode, when
the macro is defined, and once in interpretation mode, when the macro is
called.  As seen above, this fact leads to exponential growth in the
quantity of escape characters required to delay interpolation of '\n',
'\g', '\$', '\*', '\V', and '\?' at each nesting level, which can be
daunting.  GNU 'troff' offers a solution.

 -- Escape sequence: \E
     '\E' represents an escape character that is not interpreted in copy
     mode.  You can use it to ease the writing of nested macro
     definitions.

          .de M1
          .  nop \E$1
          .  de M2
          .    nop \E$1
          .    de M3
          .      nop \E$1
          \\\\..
          .    M3 better.
          \\..
          .  M2 bit
          ..
          This vehicle handles
          .M1 a
              => This vehicle handles a bit better.

     Observe that because '\.' is not a true escape sequence, we can't
     use '\E' to keep '..' from ending a macro definition prematurely.
     If the multiplicity of backslashes complicates maintenance, use end
     macros.

     '\E' is also convenient to define strings containing escape
     sequences that need to work when used in copy mode (for example, as
     macro arguments), or which will be interpolated at varying macro
     nesting depths.  We might define strings to begin and end
     superscripting as follows.(2)  (*note Copy Mode-Footnote-2::)

          .ds { \v'-.9m\s'\En[.s]*7u/10u'+.7m'
          .ds } \v'-.7m\s0+.9m'

     When the 'ec' request is used to redefine the escape character,
     '\E' also makes it easier to distinguish the semantics of an escape
     character from the other meaning(s) its character might have.
     Consider the use of an unusual escape character, '-'.

          .nr a 1
          .ec -
          .de xx
          --na
          ..
          .xx
              => -na

     This result may surprise you; some people expect '1' to be output
     since register 'a' has clearly been defined with that value.  What
     has happened?  The robotic replacement of '\' with '-' has led us
     astray.  You might recognize the sequence '--' more readily with
     the default escape character as '\-', the special character escape
     sequence for the minus sign glyph.

          .nr a 1
          .ec -
          .de xx
          -Ena
          ..
          .xx
              => 1

   (1) Compare this to the '\def' and '\edef' commands in TeX.

   (2) These are lightly adapted from the 'groff' implementation of the
'ms' macros.

5.25 Page Motions
=================

*Note Manipulating Spacing::, for a discussion of the most commonly used
request for vertical motion, 'sp', which spaces downward by one vee.

 -- Request: .mk [reg]
 -- Request: .rt [dist]
     You can "mark" a location on a page for subsequent "return".  'mk'
     takes an argument, a register name in which to store the current
     page location.  If given no argument, it stores the location in an
     internal register.  This location can be used later by the 'rt' or
     the 'sp' requests (or the '\v' escape).

     The 'rt' request returns _upward_ to the location marked with the
     last 'mk' request.  If used with an argument, it returns to a
     vertical position DIST from the top of the page (no previous call
     to 'mk' is necessary in this case).  The default scaling indicator
     is 'v'.

     If a page break occurs between a 'mk' request and its matching 'rt'
     request, the 'rt' request is silently ignored.

     A simple implementation of a macro to set text in two columns
     follows.

          .nr column-length 1.5i
          .nr column-gap 4m
          .nr bottom-margin 1m
          .
          .de 2c
          .  br
          .  mk
          .  ll \\n[column-length]u
          .  wh -\\n[bottom-margin]u 2c-trap
          .  nr right-side 0
          ..
          .
          .de 2c-trap
          .  ie \\n[right-side] \{\
          .    nr right-side 0
          .    po -(\\n[column-length]u + \\n[column-gap]u)
          .    \" remove trap
          .    wh -\\n[bottom-margin]u
          .  \}
          .  el \{\
          .    \" switch to right side
          .    nr right-side 1
          .    po +(\\n[column-length]u + \\n[column-gap]u)
          .    rt
          .  \}
          ..

     Now let us apply our two-column macro.

          .pl 1.5i
          .ll 4i
          This is a small test that shows how the
          rt request works in combination with mk.

          .2c
          Starting here, text is typeset in two columns.
          Note that this implementation isn't robust
          and thus not suited for a real two-column
          macro.
              => This is a small test that shows how the
              => rt request works in combination with mk.
              =>
              => Starting  here,    isn't    robust
              => text is typeset    and   thus  not
              => in two columns.    suited  for   a
              => Note that  this    real two-column
              => implementation     macro.

   Several escape sequences enable fine control of movement about the
page.

 -- Escape sequence: \v'expr'
     Vertically move the drawing position.  EXPR indicates the magnitude
     of motion: positive is downward and and negative upward.  The
     default scaling unit is 'v'.  The motion is relative to the current
     drawing position unless EXPR begins with the boundary-relative
     motion operator '|'.  *Note Numeric Expressions::.

     Text processing continues at the new drawing position; usually,
     vertical motions should be in balanced pairs to avoid a confusing
     page layout.

     '\v' will not spring a vertical position trap.  This can be useful;
     for example, consider a page bottom trap macro that prints a marker
     in the margin to indicate continuation of a footnote.  *Note
     Traps::.

   A few escape sequences that produce vertical motion are unusual.
They are thought to originate early in AT&T 'nroff' history to achieve
super- and subscripting by half-line motions on line printers and
teletypewriters before the phototypesetter made more precise positioning
available.  They are reckoned in ems--not vees--to maintain continuity
with their original purpose of moving relative to the size of the type
rather than the distance between text baselines (vees).(1)  (*note Page
Motions-Footnote-1::)

 -- Escape sequence: \r
 -- Escape sequence: \u
 -- Escape sequence: \d
     Move upward 1m, upward .5m, and downward .5m, respectively.

Let us see these escape sequences in use.

     Obtain 100 cm\u3\d of \ka\d\092\h'|\nau'\r233\dU.

   In the foregoing we have paired '\u' and '\d' to typeset a
superscript, and later a full em negative ("reverse") motion to place a
superscript above a subscript.  A numeral-width horizontal motion escape
sequence aligns the proton and nucleon numbers, while '\k' marks a
horizontal position to which '\h' returns so that we could stack them.
(We shall discuss these horizontal motion escape sequences presently.)
In serious applications, we often want to alter the type size of the
-scripts and to fine-tune the vertical motions, as the 'groff' 'ms'
package does with its super- and subscripting string definitions.

 -- Escape sequence: \h'expr'
     Horizontally move the drawing position.  EXPR indicates the
     magnitude of motion: positive is rightward and negative leftward.
     The default scaling unit is 'm'.  The motion is relative to the
     current drawing position unless EXPR begins with the
     boundary-relative motion operator '|'.  *Note Numeric
     Expressions::.

   The following string definition sets the TeX logo.(2)  (*note Page
Motions-Footnote-2::)

     .ds TeX T\h'-.1667m'\v'.224m'E\v'-.224m'\h'-.125m'X\"

   There are a number of special-case escape sequences for horizontal
motion.

 -- Escape sequence: \<SP>
     Move right one word space.  (The input is a backslash followed by a
     space.)  This escape sequence can be thought of as a
     non-adjustable, unbreakable space.  Usually you want '\~' instead;
     see *note Manipulating Filling and Adjustment::.

 -- Escape sequence: \|
     Move one-sixth em to the right on typesetting output devices.  If a
     glyph named '\|' is defined in the current font, its width is used
     instead, even on terminal output devices.

 -- Escape sequence: \^
     Move one-twelfth em to the right on typesetting output devices.  If
     a glyph named '\^' is defined in the current font, its width is
     used instead, even on terminal output devices.

 -- Escape sequence: \0
     Move right by the width of a numeral in the current font.

   Horizontal motions are not discarded at the end of an output line as
word spaces are.  *Note Breaking::.

 -- Escape sequence: \w'anything'
 -- Register: \n[st]
 -- Register: \n[sb]
 -- Register: \n[rst]
 -- Register: \n[rsb]
 -- Register: \n[ct]
 -- Register: \n[ssc]
 -- Register: \n[skw]
     Interpolate the width of ANYTHING in basic units.  This escape
     sequence allows several properties of formatted output to be
     measured without writing it out.

          The length of the string 'abc' is \w'abc'u.
              => The length of the string 'abc' is 72u.

     ANYTHING is processed in a dummy environment: this means that font
     and type size changes, for example, may occur within it without
     affecting subsequent output.

     After each use, '\w' sets several registers.

     'st'
     'sb'
          The maximum vertical displacements of the text baseline above
          and below, respectively.  The sign convention is opposite that
          of relative vertical motions; that is, depth below the
          (original) baseline is negative.  These registers are
          incorrectly documented in the AT&T 'troff' manual as "the
          highest and lowest extent of [the argument to '\w'] relative
          to the baseline".

     'rst'
     'rsb'
          Like 'st' and 'sb', but taking account of the heights and
          depths of glyphs.  In other words, these registers store the
          highest and lowest vertical positions attained by ANYTHING,
          doing what AT&T 'troff' documented 'st' and 'sb' as doing.

     'ct'
          Characterizes the geometry of glyphs occurring in ANYTHING.

          0
               only short glyphs, no descenders or tall glyphs

          1
               at least one descender

          2
               at least one tall glyph

          3
               at least one each of a descender and a tall glyph

     'ssc'
          The amount of horizontal space (possibly negative) that should
          be added to the last glyph before a subscript.

     'skw'
          How far to right of the center of the last glyph in the '\w'
          argument, the center of an accent from a roman font should be
          placed over that glyph.

 -- Escape sequence: \kp
 -- Escape sequence: \k(ps
 -- Escape sequence: \k[position]
     Store the current horizontal position in the _input_ line in a
     register with the name POSITION (one-character name P,
     two-character name PS).  Use this, for example, to return to the
     beginning of a string for highlighting or other decoration.

 -- Register: \n[hp]
     The current horizontal position at the input line.

 -- Register: \n[.k]
     A read-only register containing the current horizontal output
     position (relative to the current indentation).

 -- Escape sequence: \o'abc'
     Overstrike glyphs A, B, C, ...; the glyphs are centered, and the
     resulting spacing is the largest width of the affected glyphs.

 -- Escape sequence: \zc
     Print glyph C with zero width, i.e., without spacing.  Use this to
     overstrike glyphs left-aligned.

 -- Escape sequence: \Z'anything'
     Save the drawing position, format ANYTHING, then restore it.  The
     argument may not contain tabs or leaders.

     An example of a strike-through macro follows.

          .de ST
          .nr ww \w'\\$1'
          \Z@\v'-.25m'\l'\\n[ww]u'@\\$1
          ..
          .
          This is
          .ST "a test"
          an actual emergency!

   (1) At the 'grops' defaults of 10-point type on 12-point vertical
spacing, the difference between half a vee and half an em can be subtle:
large spacings like '.vs .5i' make it obvious.

   (2) *Note Strings::, for an explanation of the trailing '\"'.

5.26 Drawing Requests
=====================

'gtroff' provides a number of ways to draw lines and other figures on
the page.  Used in combination with the page motion commands (*note Page
Motions::), a wide variety of figures can be drawn.  However, for
complex drawings these operations can be quite cumbersome, and it may be
wise to use graphic preprocessors like 'gpic' or 'ggrn'.

   All drawing is done via escape sequences.

 -- Escape sequence: \l'l'
 -- Escape sequence: \l'lc'
     Draw a line horizontally.  L is the length of the line to be drawn.
     If it is positive, start the line at the current location and draw
     to the right; its end point is the new current location.  Negative
     values are handled differently: The line starts at the current
     location and draws to the left, but the current location doesn't
     move.

     L can also be specified absolutely (i.e., with a leading '|'),
     which draws back to the beginning of the input line.  Default
     scaling indicator is 'm'.

     The optional second parameter C is a glyph to draw the line with.
     If this second argument is not specified, 'gtroff' uses the
     underscore glyph, '\[ru]'.

     To separate the two arguments (to prevent 'gtroff' from
     interpreting a drawing glyph as a scaling indicator if the glyph is
     represented by a single character) use '\&'.

          .de textbox
          \[br]\\$*\[br]\l'|0\[rn]'\l'|0\[ul]'
          ..

     The above works by outputting a box rule (a vertical line), then
     the text given as an argument and then another box rule.  Finally,
     the line-drawing escape sequences both draw from the current
     location to the beginning of the _input_ line--this works because
     the line length is negative, not moving the current point.

 -- Escape sequence: \L'l'
 -- Escape sequence: \L'lg'
     Draw vertical lines.  Its parameters are similar to the '\l'
     escape, except that the default scaling indicator is 'v'.  The
     movement is downward for positive values, and upward for negative
     values.  The default glyph is the box rule glyph, '\[br]'.  As with
     the vertical motion escape sequences, text processing blindly
     continues where the line ends.

          This is a \L'3v'test.

     Here is the result, produced with 'grotty'.

          This is a
                    |
                    |
                    |test.

 -- Escape sequence: \D'command arg ...'
     The '\D' escape provides a variety of drawing functions.  On
     character devices, only vertical and horizontal lines are supported
     within 'grotty'; other devices may only support a subset of the
     available drawing functions.

     The default scaling indicator for all subcommands of '\D' is 'm'
     for horizontal distances and 'v' for vertical ones.  Exceptions are
     '\D'f ...'' and '\D't ...'', which use 'u' as the default, and
     '\D'FX ...'', which arguments are treated similar to the 'defcolor'
     request.

     '\D'l DX DY''
          Draw a line from the current location to the relative point
          specified by (DX,DY), where positive values mean right and
          down, respectively.  The end point of the line is the new
          current location.

          The following example is a macro for creating a box around a
          text string; for simplicity, the box margin is taken as a
          fixed value, 0.2m.

               .de TEXTBOX
               .  nr @wd \w'\\$1'
               \h'.2m'\
               \h'-.2m'\v'(.2m - \\n[rsb]u)'\
               \D'l 0 -(\\n[rst]u - \\n[rsb]u + .4m)'\
               \D'l (\\n[@wd]u + .4m) 0'\
               \D'l 0 (\\n[rst]u - \\n[rsb]u + .4m)'\
               \D'l -(\\n[@wd]u + .4m) 0'\
               \h'.2m'\v'-(.2m - \\n[rsb]u)'\
               \\$1\
               \h'.2m'
               ..

          First, the width of the string is stored in register '@wd'.
          Then, four lines are drawn to form a box, properly offset by
          the box margin.  The registers 'rst' and 'rsb' are set by the
          '\w' escape, containing the largest height and depth of the
          whole string.

     '\D'c D''
          Draw a circle with a diameter of D with the leftmost point at
          the current position.  After drawing, the current location is
          positioned at the rightmost point of the circle.

     '\D'C D''
          Draw a solid circle with the same parameters and behaviour as
          an outlined circle.  No outline is drawn.

     '\D'e X Y''
          Draw an ellipse with a horizontal diameter of X and a vertical
          diameter of Y with the leftmost point at the current position.
          After drawing, the current location is positioned at the
          rightmost point of the ellipse.

     '\D'E X Y''
          Draw a solid ellipse with the same parameters and behaviour as
          an outlined ellipse.  No outline is drawn.

     '\D'a DX1 DY1 DX2 DY2''
          Draw an arc clockwise from the current location through the
          two specified relative locations (DX1,DY1) and (DX2,DY2).  The
          coordinates of the first point are relative to the current
          position, and the coordinates of the second point are relative
          to the first point.  After drawing, the current position is
          moved to the final point of the arc.

     '\D'~ DX1 DY1 DX2 DY2 ...''
          Draw a spline from the current location to the relative point
          (DX1,DY1) and then to (DX2,DY2), and so on.  The current
          position is moved to the terminal point of the drawn curve.

     '\D'f N''
          Set the shade of gray to be used for filling solid objects
          to N; N must be an integer between 0 and 1000, where 0
          corresponds solid white and 1000 to solid black, and values in
          between correspond to intermediate shades of gray.  This
          applies only to solid circles, solid ellipses, and solid
          polygons.  By default, a level of 1000 is used.

          Nonintuitively, the current point is moved horizontally to the
          right by N.

          Don't use this command!  It has the serious drawback that it
          is always rounded to the next integer multiple of the
          horizontal motion quantum (the value of the 'hor' keyword in
          the 'DESC' file).  Use '\M' (*note Colors::) or '\D'Fg ...''
          instead.

     '\D'p DX1 DY1 DX2 DY2 ...''
          Draw a polygon from the current location to the relative
          position (DX1,DY1) and then to (DX2,DY2) and so on.  When the
          specified data points are exhausted, a line is drawn back to
          the starting point.  The current position is changed by adding
          the sum of all arguments with odd index to the actual
          horizontal position and the even ones to the vertical
          position.

     '\D'P DX1 DY1 DX2 DY2 ...''
          Draw a solid polygon with the same parameters and behaviour as
          an outlined polygon.  No outline is drawn.

          Here a better variant of the box macro to fill the box with
          some color.  The box must be drawn before the text since
          colors in GNU 'troff' are not transparent; the filled polygon
          would hide the text completely.

               .de TEXTBOX
               .  nr @wd \w'\\$1'
               \h'.2m'\
               \h'-.2m'\v'(.2m - \\n[rsb]u)'\
               \M[lightcyan]\
               \D'P 0 -(\\n[rst]u - \\n[rsb]u + .4m) \
                    (\\n[@wd]u + .4m) 0 \
                    0 (\\n[rst]u - \\n[rsb]u + .4m) \
                    -(\\n[@wd]u + .4m) 0'\
               \h'.2m'\v'-(.2m - \\n[rsb]u)'\
               \M[]\
               \\$1\
               \h'.2m'
               ..

          If you want a filled polygon that has exactly the same size as
          an unfilled one, you must draw both an unfilled and a filled
          polygon.  A filled polygon is always smaller than an unfilled
          one because the latter uses straight lines with a given line
          thickness to connect the polygon's corners, while the former
          simply fills the area defined by the coordinates.

               \h'1i'\v'1i'\
               \# increase line thickness
               \Z'\D't 5p''\
               \# draw unfilled polygon
               \Z'\D'p 3 3 -6 0''\
               \# draw filled polygon
               \Z'\D'P 3 3 -6 0''

     '\D't N''
          Set the current line thickness to N basic units.  A value of
          zero selects the smallest available line thickness.  A
          negative value makes the line thickness proportional to the
          current type size (this is the default behaviour of AT&T
          'troff').

          Nonintuitively, the current point is moved horizontally to the
          right by N.

     '\D'FSCHEME COLOR_COMPONENTS''
          Change current fill color.  SCHEME is a single letter denoting
          the color scheme: 'r' (rgb), 'c' (cmy), 'k' (cmyk), 'g'
          (gray), or 'd' (default color).  The color components use
          exactly the same syntax as in the 'defcolor' request (*note
          Colors::); the command '\D'Fd'' doesn't take an argument.

          _No_ position changing!

          Examples:

               \D'Fg .3'      \" same gray as \D'f 700'
               \D'Fr #0000ff' \" blue

   *Note Graphics Commands::.

 -- Escape sequence: \b'contents'
     Pile and center a sequence of glyphs vertically on the output line.
     "Piling" vertically stacks glyphs corresponding to each character
     in CONTENTS, read from left to right, and placed from top to
     bottom.  GNU 'troff' separates the glyphs vertically by 1m, and the
     pile itself is centered 0.5m above the text baseline.  The
     horizontal drawing position is then advanced by the width of the
     widest glyph in the pile.

     This rather inflexible positioning algorithm doesn't work with the
     'dvi' output device since its bracket pieces vary in height.
     Instead, use the 'geqn' preprocessor.

     *Note Manipulating Spacing::, to see how to adjust the vertical
     spacing of the output line with the '\x' escape sequence.

     The idiomatic use of '\b' is for building large brackets and
     braces, hence its name.  We might construct a large opening brace
     as follows.

          \b'\[lt]\[bv]\[lk]\[bv]\[lb]'

5.27 Deferring Output
=====================

A few 'roff' language elements are generally not used in simple
documents, but arise as page layouts become more sophisticated and
demanding.  "Environments" collect formatting parameters like line
length and typeface.  A "diversion" stores formatted output for later
use.  A "trap" is a condition on the input or output, tested
automatically by the formatter, that is associated with a macro, causing
it to be called when that condition is fulfilled.

   Footnote support often exercises all three of the foregoing features.
A simple implementation might work as follows.  A pair of macros is
defined: one starts a footnote and the other ends it.  The author calls
the first macro where a footnote marker is desired.  The macro
establishes a diversion so that the footnote text is collected at the
place in the body text where its corresponding marker appears.  An
environment is created for the footnote so that it is set at a smaller
typeface.  The footnote text is formatted in the diversion using that
environment, but it does not yet appear in the output.  The document
author calls the footnote end macro, which returns to the previous
environment and ends the diversion.  Later, after much more body text in
the document, a trap, set a small distance above the page bottom, is
sprung.  The macro called by the trap draws a line across the page and
emits the stored diversion.  Thus, the footnote is rendered.

   Diversions and traps make the text formatting process non-linear.
Let us imagine a set of text lines or paragraphs labelled 'A', 'B', and
so on.  If we set up a trap that produces text 'T' (as a page footer,
say), and we also use a diversion to store the formatted text 'D', then
a document with input text in the order 'A B C D E F' might render as 'A
B C E T F'.  The diversion 'D' will never be output if we do not call
for it.

   Environments of themselves are not a source of non-linearity in
document formatting: environment switches have immediate effect.  One
could always write a macro to change as many formatting parameters as
desired with a single convenient call.  But because diversions can be
nested and macros called by traps that are sprung by other trap-called
macros, they may be called upon in varying contexts.  For example,
consider a page header that is always to be set in Helvetica.  A
document that uses Times for most of its body text, but Courier for
displayed code examples, poses a challenge if a page break occurs in the
middle of a code display; if the header trap assumes that the "previous
font" is always Times, the rest of the example will be formatted in the
wrong typeface.  One could carefully save all formatting parameters upon
entering the trap and restore them upon leaving it, but this is verbose,
error-prone, and not future-proof as the 'groff' language develops.
Environments save us considerable effort.

5.28 Traps
==========

"Traps" are locations in the output or conditions on the input that,
when reached or fulfilled, call a specified macro.  These traps can
occur at a given location on the page, at a given location in the
current diversion (together, these are known as vertical position
traps), at a blank line, at a line with leading space characters, after
a quantity of input lines, or at the end of input.  Macros called by
traps are passed no arguments.  Setting a trap is also called "planting"
one.  It is said that a trap is "sprung" if its condition is fulfilled.

5.28.1 Vertical Position Traps
------------------------------

"Vertical position traps" perform an action when GNU 'troff' reaches or
passes a certain vertical location on the output page or in a diversion.
Their applications include setting page headers and footers, body text
in multiple columns, and footnotes.

 -- Request: .vpt [flag]
 -- Register: \n[.vpt]
     Enable vertical position traps if FLAG is non-zero or absent;
     disable them otherwise.  Vertical position traps are those set by
     the 'wh' request or by 'dt' within a diversion.  The parameter that
     controls whether vertical position traps are enabled is global.
     Initially, vertical position traps are enabled.  The current value
     is stored in the '.vpt' read-only register.

     A page can't be ejected if 'vpt' is set to zero.

5.28.1.1 Page Location Traps
............................

 -- Request: .wh dist [name]
     Call macro NAME when the vertical position DIST on the page is
     reached or passed in the downward direction.  The default scaling
     unit is 'v'.  Non-negative values for DIST set the trap relative to
     the top of the page; negative values set the trap relative to the
     bottom of the page.  An existing _visible_ trap (see below) at DIST
     is removed; this is 'wh''s sole function if NAME is missing.

     A trap is sprung only if it is "visible", meaning that its location
     is reachable on the page(1) (*note Page Location
     Traps-Footnote-1::) and it is not hidden by another trap at the
     same location already planted there.

     An example of how a macro package might set headers and footers
     follows.

          .de hd                \" page header
          '  sp .5i
          .  tl '\\*[Title]''\\*[Date]'
          '  sp .3i
          ..
          .
          .de fo                \" page footer
          '  sp 1v
          .  tl ''%''
          '  bp
          ..
          .
          .wh 0   hd            \" trap at top of the page
          .wh -1i fo            \" trap one inch from bottom

     A trap above the top or at or below the bottom of the page can be
     made visible by either moving it into the page area or increasing
     the page length so that the trap is on the page.  Negative trap
     values always use the _current_ page length; they are not converted
     to an absolute vertical position.  We can use the 'ptr' request to
     dump our page location traps to the standard error stream (*note
     Debugging::).  Their positions are reported in basic units
     appropriate to the device; an 'nroff' device example follows.

          .pl 5i
          .wh -1i xx
          .ptr
              error-> xx      -240
          .pl 100i
          .ptr
              error-> xx      -240

     It is possible to have more than one trap at the same location
     (although only one at a time can be visible); to achieve this, the
     traps must be defined at different locations, then moved to the
     same place with the 'ch' request.  In the following example, the
     many empty lines caused by the 'bp' request are not shown in the
     output.

          .de a
          .  nop a
          ..
          .de b
          .  nop b
          ..
          .de c
          .  nop c
          ..
          .
          .wh 1i a
          .wh 2i b
          .wh 3i c
          .bp
              => a b c
          .ch b 1i
          .ch c 1i
          .bp
              => a
          .ch a 0.5i
          .bp
              => a b

 -- Register: \n[.t]
     The read-only register '.t' holds the distance to the next vertical
     position trap.  If there are no traps between the current position
     and the bottom of the page, it contains the distance to the page
     bottom.  Within a diversion, in the absence of a diversion trap,
     this distance is the largest representable integer in basic
     units--effectively infinite.

 -- Request: .ch name [dist]
     Change the location of a trap by moving macro NAME to new location
     DIST, or by unplanting it altogether if DIST is absent.  The
     default scaling unit is 'v'.  Parameters to 'ch' are specified in
     the opposite order from 'wh'.  If NAME is the earliest planted
     macro of multiple traps at the same location, (re)moving it from
     that location exposes the macro next least recently planted at the
     same place.(2)  (*note Page Location Traps-Footnote-2::)

     Changing a trap's location is useful for building up footnotes in a
     diversion to allow more space at the bottom of the page for them.

   The same macro can be installed simultaneously at multiple locations;
however, only the earliest-planted instance--that has not yet been
deleted with 'wh'--will be moved by 'ch'.  The following example (using
an 'nroff' device) illustrates this behavior.(3)  (*note Page Location
Traps-Footnote-3::) Blank lines have been elided from the output.

     .de T
     Trap sprung at \\n(nlu.
     .br
     ..
     .wh 1i T
     .wh 2i T
     foo
     .sp 11i
     .bp
     .ch T 4i
     bar
     .sp 11i
     .bp
     .ch T 5i
     baz
     .sp 11i
     .bp
     .wh 5i
     .ch T 6i
     qux
     .sp 11i
         => foo
         => Trap sprung at 240u.
         => Trap sprung at 480u.
         => bar
         => Trap sprung at 480u.
         => Trap sprung at 960u.
         => baz
         => Trap sprung at 480u.
         => Trap sprung at 1200u.
         => qux
         => Trap sprung at 1440u.

 -- Register: \n[.ne]
     The read-only register '.ne' contains the amount of space that was
     needed in the last 'ne' request that caused a trap to be sprung; it
     is useful in conjunction with the '.trunc' register.  *Note Page
     Control::.

     Since the '.ne' register is set only by traps, it doesn't make
     sense to interpolate it outside of macros called by traps.

 -- Register: \n[.trunc]
     A read-only register containing the amount of vertical space
     truncated from an 'sp' request by the most recently sprung vertical
     position trap, or, if the trap was sprung by an 'ne' request, minus
     the amount of vertical motion produced by the 'ne' request.  In
     other words, at the point a trap is sprung, it represents the
     difference of what the vertical position would have been but for
     the trap, and what the vertical position actually is.

     Since the '.trunc' register is set only by traps, it doesn't make
     sense to interpolate it outside of macros called by traps.

 -- Register: \n[.pe]
     A read-only register containing 1 while a page is being ejected
     with the 'bp' request (or by the end of input), and 0 otherwise.

     In the following example, only the second call to 'x' is caused by
     'bp'.

          .de x
          \&.pe=\\n[.pe]
          .br
          ..
          .wh 1v x
          .wh 4v x
          A line.
          .br
          Another line.
          .br
              => A line.
                 .pe=0
                 Another line.

                 .pe=1

   An important fact to consider while designing macros is that
diversions and traps do not interact normally.  For example, if a trap
calls a header macro (while outputting a diversion) that tries to change
the font on the current page, the effect is not visible before the
diversion has completely been printed (except for input protected with
'\!' or '\?') since the data in the diversion is already formatted.  In
most cases, this is not the expected behaviour.

   (1) A trap planted at '20i' or '-30i' will not be sprung on a page of
length '11i'.

   (2) It may help to think of each trap location as maintaining a
queue; 'wh' operates on the head of the queue, and 'ch' operates on its
tail.  Only the trap at the head of the queue is visible.

   (3) ...which is compatible with Heirloom Doctools 'troff'.

5.28.1.2 Diversion Traps
........................

 -- Request: .dt [dist name]
     Set a trap _within_ a diversion at location DIST, which is
     interpreted relative to diversion rather than page boundaries.
     There exists only a single diversion trap per diversion.  If
     invoked with fewer than two arguments, any diversion trap in the
     current diversion is removed.  The register '.t' works within
     diversions.  It is an error to invoke 'dt' in the top-level
     diversion.  *Note Diversions::.

5.28.2 Input Line Traps
-----------------------

 -- Request: .it [n name]
 -- Request: .itc [n name]
     Set an input line trap, calling macro NAME after processing the
     next N productive input lines (*note Manipulating Filling and
     Adjustment::).  Any existing input line trap in the environment is
     replaced.  Without arguments, 'it' and 'itc' clear any input line
     trap that has not yet sprung.

     Consider a macro '.ST S N' which sets the next N input lines in the
     font style S.

          .de ST \" Use style $1 for next $2 text lines.
          .  it \\$2 ES
          .  ft \\$1
          ..
          .de ES \" end ST
          .  ft R
          ..
          .ST I 1
          oblique
          face
          .ST I 1
          oblique\c
          face
              => oblique face obliqueface  (second "face" upright)

     Unlike the 'ce' and 'rj' requests, 'it' counts lines interrupted
     with the '\c' escape sequence separately (*note Line
     Continuation::); 'itc' does not.  To see the difference, let's
     change the previous example to use 'itc' instead.

          ...
          .  itc \\$2 ES
          ...
              => oblique face obliqueface  (second "face" oblique)

     You can think of the 'ce' and 'rj' requests as implicitly creating
     an input line trap with 'itc' that schedules a break when the trap
     is sprung.

          .de BR
          .  br
          .  internal: disable centering-without-filling
          ..
          .
          .de ce
          .  if \\n[.br] .br
          .  itc \\$1 BR
          .  internal: enable centering-without-filling
          ..

     Let us consider in more detail the sorts of input lines that are or
     are not "productive".

          .de Trap
          TRAP SPRUNG
          ..
          .de Mac
          .if r a \l'5n'
          ..
          .it 2 Trap
          .
          foo
          .Mac
          bar
          baz
          .it 1 Trap
          .sp \" moves, but does not write or draw
          qux
          .itc 1 Trap
          \h'5n'\c \" moves, but does not write or draw
          jat

     When 'Trap' gets called depends on whether the 'a' register is
     defined; the control line with the 'if' request may or may not
     produce written output.  We also see that the spacing request 'sp',
     while certainly affecting the output, does not spring the input
     line trap.  Similarly, the horizontal motion escape sequence '\h'
     also affected the output, but was not "written".  Observe that we
     had to follow it with '\c' and use 'itc' to prevent the newline at
     the end of the text line from causing a word break, which, like an
     ordinary space character, counts as written output.

          $ groff -Tascii input-trap-example.groff
              => foo bar TRAP SPRUNG baz
              =>
              => qux TRAP SPRUNG      jat TRAP SPRUNG
          $ groff -Tascii -ra1 input-trap-example.groff
              => foo _____ TRAP SPRUNG bar baz
              =>
              => qux TRAP SPRUNG      jat TRAP SPRUNG

   Input line traps are associated with the environment (*note
Environments::); switching to another environment suspends the current
input line trap, and going back resumes it, restoring the count of
qualifying lines enumerated in that environment.

5.28.3 Blank Line Traps
-----------------------

 -- Request: .blm [name]
     Set a blank line trap, calling the macro NAME when GNU 'troff'
     encounters a blank line in an input file, instead of the usual
     behavior (*note Breaking::).  A line consisting only of spaces is
     also treated as blank and subject to this trap.  If no argument is
     supplied, the default blank line behavior is (re-)established.

5.28.4 Leading Space Traps
--------------------------

 -- Request: .lsm [name]
 -- Register: \n[lsn]
 -- Register: \n[lss]
     Set a leading space trap, calling the macro NAME when GNU 'troff'
     encounters leading spaces in an input line; the implicit line break
     that normally happens in this case is suppressed.  If no argument
     is supplied, the default leading space behavior is (re-)established
     (*note Breaking::).

     The count of leading spaces on an input line is stored in register
     'lsn', and the amount of corresponding horizontal motion in
     register 'lss', irrespective of whether a leading space trap is
     set.  When it is, the leading spaces are removed from the input
     line, and no motion is produced before calling NAME.

5.28.5 End-of-input Traps
-------------------------

 -- Request: .em [name]
     Set a trap at the end of input, calling macro NAME after the last
     line of the last input file has been processed.  If no argument is
     given, any existing end-of-input trap is removed.

     For example, if the document had to have a section at the bottom of
     the last page for someone to approve it, the 'em' request could be
     used.

          .de approval
          \c
          .  ne 3v
          .  sp (\\n[.t]u - 3v)
          .  in +4i
          .  lc _
          .  br
          Approved:\t\a
          .  sp
          Date:\t\t\a
          ..
          .
          .em approval

     The '\c' in the above example needs explanation.  For historical
     reasons (compatibility with AT&T 'troff'), the end-of-input macro
     exits as soon as it causes a page break if no partially collected
     line remains.(1)  (*note End-of-input Traps-Footnote-1::)

     Let us assume that there is no '\c' in the above 'approval' macro,
     that the page is full, and last output line has been broken with,
     say, a 'br' request.  Because there is no more room, a 'ne' request
     at this point causes a page ejection, which in turn makes 'troff'
     exit immediately as just described.  In most situations, this is
     not desired; people generally want to format the input after 'ne'.

     To force processing of the whole end-of-input macro independently
     of this behavior, it is thus advisable to (invisibly) ensure the
     existence of a partially collected line ('\c') whenever there is a
     chance that a page break can happen.  In the above example,
     invoking the 'ne' request ensures that there is room for the
     subsequent formatted output on the same page, so we need insert
     '\c' only once.

     The next example shows how to append three lines, then start a new
     page unconditionally.  Since '.ne 1' doesn't give the desired
     effect--there is always one line available or we are already at the
     beginning of the next page--we temporarily increase the page length
     by one line so that we can use '.ne 2'.

          .de EM
          .pl +1v
          \c
          .ne 2
          line one
          .br
          \c
          .ne 2
          line two
          .br
          \c
          .ne 2
          line three
          .br
          .pl -1v
          \c
          'bp
          ..
          .em EM

     This specific feature affects only the first potential page break
     caused by the end-of-input macro; further page breaks emitted by
     the macro are handled normally.

     Another possible use of the 'em' request is to make GNU 'troff'
     emit a single large page instead of multiple pages.  For example,
     one may want to produce a long plain text file for reading in a
     terminal or emulator without page footers and headers interrupting
     the body of the document.  One approach is to set the page length
     at the beginning of the document to a very large value to hold all
     the text,(2) (*note End-of-input Traps-Footnote-2::) and
     automatically adjust it to the exact height of the document after
     the text has been output.

          .de adjust-page-length
          .  br
          .  pl \\n[nl]u \" \n[nl]: current vertical position
          ..
          .
          .de single-page-mode
          .  pl 99999
          .  em adjust-page-length
          ..
          .
          .\" Activate the above code if configured.
          .if \n[do-continuous-rendering] \
          .  single-page-mode

     Since only one end-of-input trap exists and another macro package
     may already use it, care must be taken not to break the mechanism.
     A simple solution would be to append the above macro to the macro
     package's end-of-input macro using the 'am' request.

   (1) While processing an end-of-input macro, the formatter assumes
that the next page break must be the last; it goes into "sudden death
overtime".

   (2) Another, taken by the 'groff' 'man' macros, is to intercept 'ne'
requests and wrap 'bp' ones.

5.29 Diversions
===============

In 'roff' systems it is possible to format text as if for output, but
instead of writing it immediately, one can "divert" the formatted text
into a named storage area.  It is retrieved later by specifying its name
after a control character.  The same name space is used for such
diversions as for strings and macros; see *note Identifiers::.  Such
text is sometimes said to be "stored in a macro", but this coinage
obscures the important distinction between macros and strings on one
hand and diversions on the other; the former store _unformatted_ input
text, and the latter capture _formatted_ output.  Diversions also do not
interpret arguments.  Applications of diversions include "keeps"
(preventing a page break from occurring at an inconvenient place by
forcing a set of output lines to be set as a group), footnotes, tables
of contents, and indices.  For orthogonality it is said that GNU 'troff'
is in the "top-level diversion" if no diversion is active (that is,
formatted output is being "diverted" immediately to the output device).

   Dereferencing an undefined diversion will create an empty one of that
name and cause a warning in category 'mac' to be emitted.  *Note
Warnings::, for information about the enablement and suppression of
warnings.  A diversion does not exist for the purpose of testing with
the 'd' conditional operator until its initial definition ends (*note
Operators in Conditionals::).  The following requests are used to create
and alter diversions.

 -- Request: .di [name]
 -- Request: .da [name]
     Start collecting formatted output in a diversion called NAME.  The
     'da' request appends to a diversion called NAME, creating it if
     necessary.  If NAME already exists as an alias, the target of the
     alias is replaced or appended to; recall *note Strings::.  The
     pending output line is diverted as well.  Switching to another
     environment (with the 'ev' request) before invoking 'di' or 'da'
     avoids including any pending output line in the diversion; see
     *note Environments::.

     Invoking 'di' or 'da' without an argument stops diverting output to
     the diversion named by the most recent corresponding request.  If
     'di' or 'da' is called without an argument when there is no current
     diversion, a warning in category 'di' is produced.  *Note
     Warnings::, for information about the enablement and suppression of
     warnings.

          Before the diversion.
          .di yyy
          In the diversion.
          .br
          .di
          After the diversion.
          .br
              => After the diversion.
          .yyy
              => Before the diversion.  In the diversion.

   Because it is often desirable to exclude the partially collected line
from a diversion, 'groff' supports an alternative form of diversion
known as a "box".

 -- Request: .box [name]
 -- Request: .boxa [name]
     Divert (or append) output to NAME, similarly to the 'di' and 'da'
     requests, respectively.  Any output line pending when these
     requests are invoked is _not_ included in the box.  Calling 'box'
     or 'boxa' without an argument stops diverting output to the box
     named by the most recent corresponding request; a pending output
     line inside a diversion is discarded.

          Before the box.
          .box xxx
          In the box.
          .br
          Hidden treasure.
          .box
          After the box.
          .br
              => Before the box.  After the box.
          .xxx
              => In the box.

   Apart from pending output line inclusion and the request names that
populate them, boxes are handled exactly as diversions are.  All of the
following 'groff' language elements can be used with them
interchangeably.

 -- Register: \n[.z]
 -- Register: \n[.d]
     Diversions may be nested.  The read-only string-valued register
     '.z' contains the name of the current diversion.  The read-only
     register '.d' contains the current vertical place in the diversion.
     If the input text is not being diverted, '.d' reports the same
     location as the register 'nl'.

 -- Register: \n[.h]
     The read-only register '.h' stores the "high-water mark" on the
     current page or in the current diversion.  It corresponds to the
     text baseline of the lowest line on the page.(1)  (*note
     Diversions-Footnote-1::)

          .tm .h==\n[.h], nl==\n[nl]
              => .h==0, nl==-1
          This is a test.
          .br
          .sp 2
          .tm .h==\n[.h], nl==\n[nl]
              => .h==40, nl==120

     As implied by the example, vertical motion does not produce text
     baselines and thus does not increase the value interpolated by
     '\n[.h]'.

 -- Register: \n[dn]
 -- Register: \n[dl]
     After completing a diversion, the writable registers 'dn' and 'dl'
     contain its vertical and horizontal sizes.  Only the lines just
     processed are counted: for the computation of 'dn' and 'dl', the
     requests 'da' and 'boxa' are handled as if 'di' and 'box' had been
     used, respectively--lines that have been already stored in the
     diversion (box) are not taken into account.

          .\" Center text both horizontally and vertically.
          .\" Macro .(c starts centering mode; .)c terminates it.
          .
          .\" Disable the escape character with .eo so that we
          .\" don't have to double backslashes on the "\n"s.
          .eo
          .de (c
          .  br
          .  ev (c
          .  evc 0
          .  in 0
          .  nf
          .  di @c
          ..
          .de )c
          .  br
          .  ev
          .  di
          .  nr @s (((\n[.t]u - \n[dn]u) / 2u) - 1v)
          .  sp \n[@s]u
          .  ce 1000
          .  @c
          .  ce 0
          .  sp \n[@s]u
          .  br
          .  fi
          .  rr @s
          .  rm @c
          ..
          .ec

 -- Escape sequence: \!anything
 -- Escape sequence: \?anything\?
     "Transparently" embed ANYTHING into the current diversion,
     preventing requests, macro calls, and escape sequences from being
     interpreted when read into a diversion.  This is useful for
     preventing them from taking effect until the diverted text is
     actually output.  The '\!' escape sequence transparently embeds
     input up to and including the end of the line.  The '\?' escape
     sequence transparently embeds input until its own next occurrence.

     ANYTHING may not contain newlines; use '\!' by itself to embed
     newlines in a diversion.  The escape sequence '\?' is also
     recognized in copy mode and turned into a single internal code; it
     is this code that terminates ANYTHING.  Thus the following example
     prints 4.

          .nr x 1
          .nf
          .di d
          \?\\?\\\\?\\\\\\\\nx\\\\?\\?\?
          .di
          .nr x 2
          .di e
          .d
          .di
          .nr x 3
          .di f
          .e
          .di
          .nr x 4
          .f

     Both escape sequences read the data in copy mode.

     If '\!' is used in the top-level diversion, its argument is
     directly embedded into GNU 'troff''s intermediate output.  This can
     be used, for example, to control a postprocessor that processes the
     data before it is sent to an output driver.

     The '\?' escape used in the top-level diversion produces no output
     at all; its argument is simply ignored.

 -- Request: .output contents
     Emit CONTENTS directly to GNU 'troff''s intermediate output
     (subject to copy mode interpretation); this is similar to '\!' used
     at the top level.  An initial neutral double quote in CONTENTS is
     stripped to allow embedding of leading spaces.

     This request can't be used before the first page has started--if
     you get an error, simply insert '.br' before the 'output' request.

     Use with caution!  It is normally only needed for mark-up used by a
     postprocessor that does something with the output before sending it
     to the output device, filtering out CONTENTS again.

 -- Request: .asciify div
     "Unformat" the diversion DIV in a way such that Unicode basic Latin
     (ASCII) characters, characters translated with the 'trin' request,
     space characters, and some escape sequences, that were formatted
     and diverted into DIV are treated like ordinary input characters
     when DIV is reread.  Doing so can be useful in conjunction with the
     'writem' request.  'asciify' can be also used for gross hacks; for
     example, the following sets register 'n' to 1.

          .tr @.
          .di x
          @nr n 1
          .br
          .di
          .tr @@
          .asciify x
          .x

     'asciify' cannot return all items in a diversion to their source
     equivalent: nodes such as those produced by the '\N' escape
     sequence will remain nodes, so the result cannot be guaranteed to
     be a pure string.  *Note Copy Mode::.  Glyph parameters such as the
     type face and size are not preserved; use 'unformat' to achieve
     that.

 -- Request: .unformat div
     Like 'asciify', unformat the diversion DIV.  However, 'unformat'
     handles only tabs and spaces between words, the latter usually
     arising from spaces or newlines in the input.  Tabs are treated as
     input tokens, and spaces become adjustable again.  The vertical
     sizes of lines are not preserved, but glyph information (font, type
     size, space width, and so on) is retained.

   (1) Thus, the "water" gets "higher" proceeding _down_ the page.

5.30 Punning Names
==================

Macros, strings, and diversions share a name space; recall *note
Identifiers::.  Internally, the same mechanism is used to store them.
You can thus call a macro with string interpolation syntax and vice
versa.

     .de subject
     Typesetting
     ..
     .de predicate
     rewards attention to detail
     ..
     \*[subject] \*[predicate].
     Truly.
         => Typesetting
         =>  rewards attention to detail Truly.

What went wrong?  Strings don't contain newlines, but macros do.  String
interpolation placed a newline at the end of '\*[subject]', and the next
thing on the input was a space.  Then when '\*[predicate]' was
interpolated, it was followed by the empty request '.' on a line by
itself.  If we want to use macros as strings, we must take interpolation
behavior into account.

     .de subject
     Typesetting\\
     ..
     .de predicate
     rewards attention to detail\\
     ..
     \*[subject] \*[predicate].
     Truly.
         => Typesetting rewards attention to detail.  Truly.

By ending each text line of the macros with an escaped '\<RET>', we get
the desired effect (*note Line Continuation::).(1)  (*note Punning
Names-Footnote-1::) What would have happened if we had used only one
backslash at a time instead?

   Interpolating a string does not hide existing macro arguments.  We
can also place the escaped newline outside the string interpolation
instead of within the string definition.  Thus, in a macro, a more
efficient way of doing

     .xx \\$@

is

     \\*[xx]\\

The latter calling syntax doesn't change the value of '\$0', which is
then inherited from the calling macro (*note Parameters::).

   Diversions can be also called with string syntax.  It is sometimes
convenient to copy one-line diversions to a string.

     .di xx
     the
     .ft I
     interpolation system
     .ft
     .br
     .di
     .ds yy This is a test of \*(xx\c
     \*(yy.
         => This is a test of the interpolation system.

As the previous example shows, it is possible to store formatted output
in strings.  The '\c' escape sequence prevents the subsequent newline
from being interpreted as a break (again, *note Line Continuation::).

   Copying multi-output line diversions produces unexpected results.

     .di xxx
     a funny
     .br
     test
     .br
     .di
     .ds yyy This is \*[xxx]\c
     \*[yyy].
         => test This is a funny.

   Usually, it is not predictable whether a diversion contains one or
more output lines, so this mechanism should be avoided.  With AT&T
'troff', this was the only solution to strip off a final newline from a
diversion.  Another disadvantage is that the spaces in the copied string
are already formatted, preventing their adjustment.  This can cause ugly
results.

   A clean solution to this problem is available in GNU 'troff', using
the requests 'chop' to remove the final newline of a diversion, and
'unformat' to make the horizontal spaces adjustable again.

     .box xxx
     a funny
     .br
     test
     .br
     .box
     .chop xxx
     .unformat xxx
     This is \*[xxx].
         => This is a funny test.

   *Note Gtroff Internals::.

   (1) The backslash is doubled.  *Note Copy Mode::.

5.31 Environments
=================

As discussed in *note Deferring Output::, environments store most of the
parameters that control text processing.  A default environment named
'0' exists when GNU 'troff' starts up; it is modified by
formatting-related requests and escape sequences.

   You can create new environments and switch among them.  Only one is
current at any given time.  Active environments are managed using a
"stack", a data structure supporting "push" and "pop" operations.  The
current environment is at the top of the stack.  The same environment
name can be pushed onto the stack multiple times, possibly interleaved
with others.  Popping the environment stack does not destroy the current
environment; it remains accessible by name and can be made current again
by pushing it at any time.  Environments cannot be renamed or deleted,
and can only be modified when current.  To inspect the environment
stack, use the 'pev' request; see *note Debugging::.

   Environments store the following information.

   * a partially collected line, if any

   * data about the most recently output glyph and line (registers
     '.cdp', '.cht', '.csk', '.n', '.w')

   * typeface parameters (size, family, style, height and slant,
     inter-word and inter-sentence space sizes)

   * page parameters (line length, title length, vertical spacing, line
     spacing, indentation, line numbering, centering, right-alignment,
     underlining, hyphenation parameters)

   * filling enablement; adjustment enablement and mode

   * tab stops; tab, leader, escape, control, no-break control,
     hyphenation, and margin characters

   * input line traps

   * stroke and fill colors

 -- Request: .ev [ident]
 -- Register: \n[.ev]
     Enter the environment IDENT, which is created if it does not
     already exist, using the same parameters as for the default
     environment used at startup.  With no argument, GNU 'troff'
     switches to the previous environment.

     Invoking 'ev' with an argument puts environment IDENT onto the top
     of the environment stack.  (If it isn't already present in the
     stack, this is a proper push.)  Without an argument, 'ev' pops the
     environment stack, making the previous environment current.  It is
     an error to pop the environment stack with no previous environment
     available.  The read-only string-valued register '.ev' contains the
     name of the current environment--the one at the top of the stack.

          .ev footnote-env
          .fam N
          .ps 6
          .vs 8
          .ll -.5i
          .ev

          ...

          .ev footnote-env
          \[dg] Observe the smaller text and vertical spacing.
          .ev

     We can familiarize ourselves with stack behavior by wrapping the
     'ev' request with a macro that reports the contents of the '.ev'
     register to the standard error stream.

          .de EV
          .  ev \\$1
          .  tm environment is now \\n[.ev]
          ..
          .
          .EV foo
          .EV bar
          .EV
          .EV baz
          .EV
          .EV
          .EV

              error-> environment is now foo
              error-> environment is now bar
              error-> environment is now foo
              error-> environment is now baz
              error-> environment is now foo
              error-> environment is now 0
              error-> error: environment stack underflow
              error-> environment is now 0

 -- Request: .evc environment
     Copy the contents of ENVIRONMENT to the current environment.

     The following environment data are not copied.

        * a partially collected line, if present;

        * the interruption status of the previous input line (due to use
          of the '\c' escape sequence);

        * the count of remaining lines to center, to right-justify, or
          to underline (with or without underlined spaces)--these are
          set to zero;

        * the activation status of temporary indentation;

        * input line traps and their associated data;

        * the activation status of line numbering (which can be
          reactivated with '.nm +0'); and

        * the count of consecutive hyphenated lines (set to zero).

 -- Register: \n[.w]
 -- Register: \n[.cht]
 -- Register: \n[.cdp]
 -- Register: \n[.csk]
     The '\n[.w]' register contains the width of the last glyph
     formatted in the environment.

     The '\n[.cht]' register contains the height of the last glyph
     formatted in the environment.

     The '\n[.cdp]' register contains the depth of the last glyph
     formatted in the environment.  It is positive for glyphs extending
     below the baseline.

     The '\n[.csk]' register contains the "skew" (how far to the right
     of the glyph's center that GNU 'troff' should place an accent) of
     the last glyph formatted in the environment.

 -- Register: \n[.n]
     The '\n[.n]' register contains the length of the previous output
     line emitted in the environment.

5.32 Suppressing Output
=======================

 -- Escape sequence: \O[num]
     Suppress GNU 'troff' output of glyphs and geometric primitives.
     The sequences '\O2', '\O3', '\O4', and '\O5' are intended for
     internal use by 'grohtml'.

     '\O0'
          Disable the emission of glyphs and geometric primitives to the
          output driver, provided that this sequence occurs at the
          outermost level (see '\O3' and '\04' below).  Horizontal
          motions corresponding to non-overstruck glyph widths still
          occur.

     '\O1'
          Enable the emission of glyphs and geometric primitives to the
          output driver, provided that this sequence occurs at the
          outermost level.

     '\O0' and '\O1' also reset the four registers 'opminx', 'opminy',
     'opmaxx', and 'opmaxy' to -1.  These four registers mark the top
     left and bottom right hand corners of a box encompassing all
     written glyphs.

     '\O2'
          Provided that this sequence occurs at the outermost level,
          enable emission of glyphs and geometric primitives, and write
          to the standard error stream the page number and values of the
          four aforementioned registers encompassing glyphs written
          since the last interpolation of a '\O' sequence, as well as
          the page offset, line length, image file name (if any),
          horizontal and vertical device motion quanta, and input file
          name.  Numeric values are in basic units.

     '\O3'
          Begin a nesting level.  This is an internal mechanism for
          'grohtml' while producing images.  At startup, 'gtroff' is at
          the outermost level.  These sequences are generated when
          processing the source document with 'pre-grohtml', which uses
          'gtroff' with the 'ps' output device, Ghostscript, and the PNM
          tools to produce images in PNG format.  They start a new page
          if the device is not 'html' or 'xhtml', to reduce the
          possibility of images crossing a page boundary.

     '\O4'
          End a nesting level.

     '\O[5PFILE]'
          Provided that this sequence occurs at the outermost level,
          write the name 'file' to the standard error stream at position
          P, which must be one of 'l', 'r', 'c', or 'i', corresponding
          to left, right, centered, and inline alignments within the
          document, respectively.  FILE is a name associated with the
          production of the next image.

 -- Register: \n[.O]
     Output suppression nesting level applied by '\O3' and '\O4' escape
     sequences.

5.33 I/O
========

'gtroff' has several requests for including files:

 -- Request: .so file
 -- Request: .soquiet file
     Replace the 'so' request's control line with the contents of the
     file named by the argument, "sourcing" it.  FILE is sought in the
     directories specified by '-I' command-line option.  If FILE does
     not exist, a warning in category 'file' is produced and the request
     has no further effect.  *Note Warnings::, for information about the
     enablement and suppression of warnings.

     'so' can be useful for large documents; e.g., allowing each chapter
     of a book to be kept in a separate file.  However, files
     interpolated with 'so' are not preprocessed; to overcome this
     limitation, see the 'gsoelim(1)' man page.

     Since GNU 'troff' replaces the entire control line with the
     contents of a file, it matters whether 'file' is terminated with a
     newline or not.  Assume that file 'xxx' contains only the word
     'foo' without a trailing newline.

          $ printf 'foo' > xxx

          The situation is
          .so xxx
          bar.
              => The situation is foobar.

     'soquiet' works the same way, except that no warning diagnostic is
     issued if FILE does not exist.

 -- Request: .pso command
     Read the standard output from the specified COMMAND and include it
     in place of the 'pso' request.

     It is an error to use this request in safer mode, which is the
     default.  Invoke GNU 'troff' or a front end with the '-U' option to
     enable unsafe mode.

     The comment regarding a final newline for the 'so' request is valid
     for 'pso' also.

 -- Request: .mso file
 -- Request: .msoquiet file
     Identical to the 'so' and 'soquiet' requests, respectively, except
     that 'gtroff' searches for the specified FILE in the same
     directories as macro files for the '-m' command-line option.  If
     the file name to be included has the form 'NAME.tmac' and it isn't
     found, these requests try to include 'tmac.NAME' and vice versa.

 -- Request: .trf file
 -- Request: .cf file
     Transparently output the contents of FILE.  Each line is output as
     if it were preceded by '\!'; however, the lines are _not_ subject
     to copy mode interpretation.  If the file does not end with a
     newline, then a newline is added ('trf' only).  For example, to
     define a macro 'x' containing the contents of file 'f', use

          .ev 1
          .di x
          .trf f
          .di
          .ev

     The calls to 'ev' prevent the partially collected output line from
     becoming part of the diversion (*note Diversions::).

     Both 'trf' and 'cf', when used in a diversion, embed a node (*note
     Gtroff Internals::) in it that, when reread, causes the contents of
     FILE to be transparently copied to the output.  In AT&T 'troff',
     the contents of FILE are immediately copied through to the output
     regardless of whether there is a current diversion; this behaviour
     is so anomalous that it must be considered a bug.

     While 'cf' copies the contents of FILE completely unprocessed,
     'trf' disallows characters such as NUL that are not valid 'gtroff'
     input characters (*note Identifiers::).

     For 'cf', within a diversion, 'completely unprocessed' means that
     each line of a file to be inserted is handled as if it were
     preceded by '\!\\!'.

     Both requests cause a line break.

 -- Request: .nx [file]
     Force 'gtroff' to continue processing of the file specified as an
     argument.  If no argument is given, immediately jump to the end of
     file.

 -- Request: .rd [prompt [arg1 arg2 ...]]
     Read from standard input, and include what is read as though it
     were part of the input file.  Text is read until a blank line is
     encountered.

     If standard input is a TTY input device (keyboard), write PROMPT to
     standard error, followed by a colon (or send BEL for a beep if no
     argument is given).

     Arguments after PROMPT are available for the input.  For example,
     the line

          .rd data foo bar

     with the input 'This is \$2.' prints

          This is bar.

   Using the 'nx' and 'rd' requests, it is easy to set up form letters.
The form letter template is constructed like this, putting the following
lines into a file called 'repeat.let':

     .ce
     \*(td
     .sp 2
     .nf
     .rd
     .sp
     .rd
     .fi
     Body of letter.
     .bp
     .nx repeat.let

When this is run, a file containing the following lines should be
redirected in.  Requests included in this file are executed as though
they were part of the form letter.  The last block of input is the 'ex'
request, which tells GNU 'troff' to stop processing.  If this were not
there, 'troff' would not know when to stop.

     Trent A. Fisher
     708 NW 19th Av., #202
     Portland, OR  97209

     Dear Trent,

     Len Adollar
     4315 Sierra Vista
     San Diego, CA  92103

     Dear Mr. Adollar,

     .ex

 -- Request: .pi pipe
     Pipe the output of 'gtroff' to the shell command(s) specified by
     PIPE.  This request must occur before 'gtroff' has a chance to
     print anything.

     It is an error to use this request in safer mode, which is the
     default.  Invoke GNU 'troff' or a front end with the '-U' option to
     enable unsafe mode.

     Multiple calls to 'pi' are allowed, acting as a chain.  For
     example,

          .pi foo
          .pi bar
          ...

     is the same as '.pi foo | bar'.

     The intermediate output format of GNU 'troff' is piped to the
     specified commands.  Consequently, calling 'groff' without the '-Z'
     option normally causes a fatal error.

 -- Request: .sy cmds
 -- Register: \n[systat]
     Execute the shell command(s) specified by CMDS.  The output is not
     saved anywhere, so it is up to the user to do so.

     It is an error to use this request in safer mode; this is the
     default.  Give GNU 'troff' or a front end program the '-U' option
     to enable unsafe mode.

     The following code fragment introduces the current time into a
     document.

          .sy perl -e 'printf ".nr H %d\\n.nr M %d\\n.nr S %d\\n",\
                       (localtime(time))[2,1,0]' > /tmp/x\n[$$]
          .so /tmp/x\n[$$]
          .sy rm /tmp/x\n[$$]
          \nH:\nM:\nS

     This works by having the Perl script (run by 'sy') write 'nr'
     requests that set the registers 'H', 'M', and 'S' to a temporary
     file.  The 'roff' document then reads the temporary file using the
     'so' request.

     The registers 'seconds', 'minutes', and 'hours', initialized at
     startup of GNU 'troff', should satisfy most requirements.  Use the
     'af' request to format their values for output.

          .af hours 00
          .af minutes 00
          .af seconds 00
          \n[hours]:\n[minutes]:\n[seconds]
              => 02:17:54

     The writable register 'systat' contains the return value of the
     'system()' function executed by the last 'sy' request.

 -- Request: .open stream file
 -- Request: .opena stream file
     Open the specified FILE for writing and associates the specified
     STREAM with it.

     The 'opena' request is like 'open', but if the file exists, append
     to it instead of truncating it.

     It is an error to use these requests in safer mode; this is the
     default.  Give GNU 'troff' or a front end program the '-U' option
     to enable unsafe mode.

 -- Request: .write stream data
 -- Request: .writec stream data
     Write to the file associated with the specified STREAM.  The stream
     must previously have been the subject of an open request.  The
     remainder of the line is interpreted as the 'ds' request reads its
     second argument: an initial neutral double quote in CONTENTS is
     stripped to allow embedding of leading spaces, and it is read in
     copy mode.

     The 'writec' request is like 'write', but only 'write' appends a
     newline to the data.

 -- Request: .writem stream xx
     Write the contents of the macro or string XX to the file associated
     with the specified STREAM.

     XX is read in copy mode, i.e., already formatted elements are
     ignored.  Consequently, diversions must be unformatted with the
     'asciify' request before calling 'writem'.  Usually, this means a
     loss of information.

 -- Request: .close stream
     Close the specified STREAM; the stream is no longer an acceptable
     argument to the 'write' request.

     Here a simple macro to write an index entry.

          .open idx test.idx
          .
          .de IX
          .  write idx \\n[%] \\$*
          ..
          .
          .IX test entry
          .
          .close idx

 -- Escape sequence: \Ve
 -- Escape sequence: \V(ev
 -- Escape sequence: \V[env]
     Interpolate the contents of the specified environment variable ENV
     (one-character name E, two-character name EV) as returned by the
     function 'getenv'.  '\V' is interpreted even in copy mode (*note
     Copy Mode::).

5.34 Postprocessor Access
=========================

Two escape sequences and two requests enable documents to pass
information directly to a postprocessor.  These are useful for
exercising device-specific capabilities that the 'groff' language does
not abstract or generalize; examples include the embedding of hyperlinks
and image files.  Device-specific functions are documented in each
output driver's man page, such as 'gropdf(1)', 'grops(1)', or
'grotty(1)'.

 -- Request: .device xxx ...
 -- Escape sequence: \X'xxx ...'
     Embed all XXX arguments into GNU 'troff' output as parameters to a
     device control command 'x X'.  The meaning and interpretation of
     such parameters is determined by the output driver or other
     postprocessor.

     The 'device' request processes its arguments in copy mode (*note
     Copy Mode::).  An initial neutral double quote in CONTENTS is
     stripped to allow embedding of leading spaces.  By contrast, within
     '\X' arguments, the escape sequences '\&', '\)', '\%', and '\:' are
     ignored; '\SP' and '\~' are converted to single space characters;
     and '\\' has its escape character stripped.  So that the basic
     Latin subset of the Unicode character set(1) (*note Postprocessor
     Access-Footnote-1::) can be reliably encoded in device control
     commands, seven special character escape sequences ('\-', '\aq',
     '\dq', '\ga', '\ha', '\rs', and '\ti',) are mapped to basic Latin
     glyphs; see the 'groff_char(7)' man page.  The use of any other
     escape sequence in '\X' arguments is normally an error.

     If the 'use_charnames_in_special' directive appears in the output
     device's 'DESC' file, the use of special character escape sequences
     is _not_ an error; they are simply output verbatim (with the
     exception of the seven mapped to Unicode basic Latin characters,
     discussed above).  'use_charnames_in_special' is currently employed
     only by 'grohtml'.

 -- Request: .devicem name
 -- Escape sequence: \Yn
 -- Escape sequence: \Y(nm
 -- Escape sequence: \Y[name]
     This is approximately equivalent to '\X'\*[NAME]'' (one-character
     name N, two-character name NM).  However, the contents of the
     string or macro NAME are not interpreted; also it is permitted for
     NAME to have been defined as a macro and thus contain newlines (it
     is not permitted for the argument to '\X' to contain newlines).
     The inclusion of newlines requires an extension to the AT&T 'troff'
     output format, and confuses drivers that do not know about this
     extension (*note Device Control Commands::).

 -- Request: .tag name
 -- Request: .taga name
     Reserved for internal use.

   (1) that is, ISO 646:1991-IRV or, popularly, "US-ASCII"

5.35 Miscellaneous
==================

This section documents parts of 'gtroff' that cannot (yet) be
categorized elsewhere in this manual.

 -- Request: .nm [start [inc [space [indent]]]]
 -- Register: \n[ln]
 -- Register: \n[.nm]
     Print line numbers.  START is the line number of the _next_ output
     line.  INC indicates which line numbers are printed.  For example,
     the value 5 means to emit only line numbers that are multiples
     of 5; this defaults to 1.  SPACE is the space to be left between
     the number and the text; this defaults to one digit space.  The
     fourth argument is the indentation of the line numbers, defaulting
     to zero.  Both SPACE and INDENT are given as multiples of digit
     spaces; they can be negative also.  Without any arguments, line
     numbers are turned off.

     'gtroff' reserves three digit spaces for the line number (which is
     printed right-justified) plus the amount given by INDENT; the
     output lines are concatenated to the line numbers, separated by
     SPACE, and _without_ reducing the line length.  Depending on the
     value of the horizontal page offset (as set with the 'po' request),
     line numbers that are longer than the reserved space stick out to
     the left, or the whole line is moved to the right.

     Parameters corresponding to missing arguments are not changed; any
     non-digit argument (to be more precise, any argument starting with
     a character valid as a delimiter for identifiers) is also treated
     as missing.

     If line numbering has been disabled with a call to 'nm' without an
     argument, it can be reactivated with '.nm +0', using the previously
     active line numbering parameters.

     The parameters of 'nm' are associated with the environment (*note
     Environments::).

     While line numbering is enabled, the output line number register
     'ln' is updated as each line is output, even if no line number is
     formatted with it because it is being skipped (it is not a multiple
     of INC) or because numbering is suppressed (see the 'nn' request
     below).

     The '.nm' register tracks the enablement status of line numbering.
     Temporary suspension of numbering with the 'nn' request does _not_
     alter its value.

          .po 1m
          .ll 2i
          This test shows how line numbering works with groff.
          .nm 999
          This test shows how line numbering works with groff.
          .br
          .nm xxx 3 2
          .ll -\w'0'u
          This test shows how line numbering works with groff.
          .nn 2
          This test shows how line numbering works with groff.

     The result is as follows.

           This  test shows how
           line numbering works
           999 with   groff.   This
          1000 test shows how  line
          1001 numbering works with
          1002 groff.
                This test shows how
                line      numbering
           works  with  groff.
           This test shows how
          1005  line      numbering
                works with groff.

 -- Request: .nn [skip]
 -- Register: \n[.nn]
     Suppress numbering of the next SKIP output lines that would
     otherwise be numbered.  The default is 1.  'nn' can be invoked when
     line numbering is not active; suppression of numbering will take
     effect for SKIP lines once 'nm' enables it.

     The '.nn' register stores the count of output lines still to have
     their numbering suppressed.

     This count is associated with the environment (*note
     Environments::).

   To test whether the current output line will be numbered, you must
check both the '.nm' and '.nn' registers.

       .de is-numbered
       .  nop This line
       .  ie (\\n[.nm] & (1-\\n[.nn])) IS
       .  el                           ISN'T
       .  nop numbered.
       .  br
       ..
       Test line numbering.
       .is-numbered
       .nm 1
       .nn 2
       .is-numbered
       .is-numbered
       .is-numbered
       .nm
       .is-numbered

The output lines correctly report their numbering status.

       Test line numbering.  This line ISN'T numbered.
       This line ISN'T numbered.
       This line ISN'T numbered.
         1 This line IS numbered.
       This line ISN'T numbered.

 -- Request: .mc glyph [dist]
     Print a "margin character" to the right of the text.(1)  (*note
     Miscellaneous-Footnote-1::) The first argument is the glyph to be
     printed.  The second argument is the distance away from the right
     margin.  If missing, the previously set value is used; the default
     is 10 points.  For text lines that are too long (that is, longer
     than the text length plus DIST), the margin character is directly
     appended to the lines.

     With no arguments the margin character is turned off.  If this
     occurs before a break, no margin character is printed.

     For compatibility with AT&T 'troff', a call to 'mc' to set the
     margin character can't be undone immediately; at least one line
     gets a margin character.  Thus

          .ll 1i
          .mc \[br]
          .mc
          xxx
          .br
          xxx

     produces

          xxx        |
          xxx

     For empty lines and lines produced by the 'tl' request no margin
     character is emitted.

     The margin character is associated with the environment (*note
     Environments::).

     This is quite useful for indicating text that has changed, and, in
     fact, there are programs available for doing this (they are called
     'nrchbar' and 'changebar' and can be found in any
     'comp.sources.unix' archive).

          .ll 3i
          .mc |
          This paragraph is highlighted with a margin
          character.
          .sp
          Vertical space isn't marked.
          .br
          \&
          .br
          But we can fake it with `\&'.

     Result:

          This  paragraph is highlighted |
          with a margin character.       |

          Vertical space isn't marked.   |
                                         |
          But we can fake it with `\&'.  |

 -- Request: .psbb filename
 -- Register: \n[llx]
 -- Register: \n[lly]
 -- Register: \n[urx]
 -- Register: \n[ury]
     Retrieve the bounding box of the PostScript image found in
     FILENAME.  The file must conform to Adobe's "Document Structuring
     Conventions" (DSC); the command searches for a '%%BoundingBox'
     comment and extracts the bounding box values into the registers
     'llx', 'lly', 'urx', and 'ury'.  If an error occurs (for example,
     'psbb' cannot find the '%%BoundingBox' comment), it sets the four
     registers to zero.

     The search path for FILENAME can be controlled with the '-I'
     command-line option.

   (1) "Margin character" is a misnomer since it is an output glyph.

5.36 'gtroff' Internals
=======================

'gtroff' processes input in three steps.  One or more input characters
are converted to an "input token".(1)  (*note Gtroff
Internals-Footnote-1::) Then, one or more input tokens are converted to
an "output node".  Finally, output nodes are converted to the
intermediate output language understood by all output devices.

   Actually, before step one happens, 'gtroff' converts certain escape
sequences into reserved input characters (not accessible by the user);
such reserved characters are used for other internal processing also -
this is the very reason why not all characters are valid input.  *Note
Identifiers::, for more on this topic.

   For example, the input string 'fi\[:u]' is converted into a character
token 'f', a character token 'i', and a special token ':u' (representing
u umlaut).  Later on, the character tokens 'f' and 'i' are merged to a
single output node representing the ligature glyph 'fi' (provided the
current font has a glyph for this ligature); the same happens with ':u'.
All output glyph nodes are 'processed', which means that they are
invariably associated with a given font, font size, advance width, etc.
During the formatting process, 'gtroff' itself adds various nodes to
control the data flow.

   Macros, diversions, and strings collect elements in two chained
lists: a list of input tokens that have been passed unprocessed, and a
list of output nodes.  Consider the following diversion.

     .di xxx
     a
     \!b
     c
     .br
     .di

It contains these elements.

node list            token list   element number
                                  
line start node      --           1
glyph node 'a'       --           2
word space node      --           3
--                   'b'          4
--                   '\n'         5
glyph node 'c'       --           6
vertical size node   --           7
vertical size node   --           8
--                   '\n'         9

Elements 1, 7, and 8 are inserted by 'gtroff'; the latter two (which are
always present) specify the vertical extent of the last line, possibly
modified by '\x'.  The 'br' request finishes the pending output line,
inserting a newline input token, which is subsequently converted to a
space when the diversion is reread.  Note that the word space node has a
fixed width that isn't adjustable anymore.  To convert horizontal space
nodes back to input tokens, use the 'unformat' request.

   Macros only contain elements in the token list (and the node list is
empty); diversions and strings can contain elements in both lists.

   Note that the 'chop' request simply reduces the number of elements in
a macro, string, or diversion by one.  Exceptions are "compatibility
save" and "compatibility ignore" input tokens, which are ignored.  The
'substring' request also ignores those input tokens.

   Some requests like 'tr' or 'cflags' work on glyph identifiers only;
this means that the associated glyph can be changed without destroying
this association.  This can be very helpful for substituting glyphs.  In
the following example, we assume that glyph 'foo' isn't available by
default, so we provide a substitution using the 'fchar' request and map
it to input character 'x'.

     .fchar \[foo] foo
     .tr x \[foo]

Now let us assume that we install an additional special font 'bar' that
has glyph 'foo'.

     .special bar
     .rchar \[foo]

Since glyphs defined with 'fchar' are searched before glyphs in special
fonts, we must call 'rchar' to remove the definition of the fallback
glyph.  Anyway, the translation is still active; 'x' now maps to the
real glyph 'foo'.

   Macro and request arguments preserve compatibility mode enablement.

     .cp 1     \" switch to compatibility mode
     .de xx
     \\$1
     ..
     .cp 0     \" switch compatibility mode off
     .xx caf\['e]
         => caf

Since compatibility mode is enabled while 'de' is invoked, the macro
'xx' enables compatibility mode when it is called.  Argument '$1' can
still be handled properly because it inherits the compatibility mode
enablement status that was active at the point where 'xx' was called.

   After interpolation of the parameters, the compatibility save and
restore tokens are removed.

   (1) Except the escape sequences '\f', '\F', '\H', '\m', '\M', '\R',
'\s', and '\S', which are processed immediately if not in copy mode.

5.37 Debugging
==============

          Standard troff voodoo, just put a power of two backslashes in
    front of it until it works and if you still have problems add a \c.
                                                         -- Ron Natalie

   GNU 'troff' is not the easiest language to debug, in part thanks to
its design features of recursive interpolation and the use of
multi-stage pipeline processing in the surrounding system.  Nevertheless
there exist several features useful for troubleshooting.

   Preprocessors use the 'lf' request to preserve the identity of the
line numbers and names of input files.  GNU 'troff' emits a variety of
error diagnostics and supports several categories of warning; the output
of these can be selectively suppressed.  Backtraces can be enabled when
errors or warnings occur, or triggered on demand.  The 'tm' and related
requests can be used to emit customized diagnostic messages or for
instrumentation while troubleshooting.  The 'ex' and 'ab' requests cause
early termination with successful and error exit codes respectively, to
halt further processing when continuing would be fruitless.  The state
of the formatter can be examined with requests that write lists of
defined names (macros, strings, and diversions), environments,
registers, and page location traps to the standard error stream.

 -- Request: .lf line [filename]
     Change the line number and optionally the file name GNU 'troff'
     shall use for error and warning messages.  LINE is the input line
     number of the _next_ line.  Without an argument, the request is
     ignored.

     This request is primarily a debugging aid for documents that
     undergo preprocessing.  Programs like 'tbl' that transform input in
     their own languages to 'roff' requests use it so that any
     diagnostic messages emitted by 'troff' correspond to the original
     source document.

 -- Request: .tm contents
 -- Request: .tm1 contents
 -- Request: .tmc contents
     Send CONTENTS, which consumes the remainder of the input line, to
     the standard error stream.

     The 'tm' request ignores leading spaces of CONTENTS; 'tm1' handles
     its argument similarly to the 'ds' request: an initial neutral
     double quote is stripped to allow embedding of leading spaces.

     The 'tmc' request is similar to 'tm1' but does not append a newline
     (as is done in 'tm' and 'tm1').

 -- Request: .ab [contents]
     Write any CONTENTS to the standard error stream (like 'tm') and
     then abort GNU 'troff'; that is, stop processing and terminate with
     a failure status.

 -- Request: .ex
     Exit GNU 'troff'; that is, stop processing and terminate with a
     successful status.  To stop processing only the current file, use
     the 'nx' request; see *note I/O::.

   When doing something involved, it is useful to leave the debugging
statements in the code and have them turned on by a command-line flag.

     .if \n[DB] .tm debugging output

To activate such statements, use the '-r' option to set the register.

     groff -rDB=1 file

   If it is known in advance that there are many errors and no useful
output, GNU 'troff' can be forced to suppress formatted output with the
'-z' option.

 -- Request: .pev
     Report the state of the current environment followed by that of all
     other environments to the standard error stream.

 -- Request: .pm
     Report, to the standard error stream, the names of all defined
     macros, strings, and diversions with their sizes in bytes.  Since
     GNU 'troff' sometimes adds nodes by itself, the returned sizes can
     be larger than expected.

 -- Request: .pnr
     Report the names and contents of all currently defined registers to
     the standard error stream.

 -- Request: .ptr
     Report the names and positions of all page location traps to the
     standard error stream.  Empty slots in the list, where a trap has
     been planted but subsequently (re)moved, are printed as well.

 -- Request: .fl
     Instruct 'gtroff' to flush its output immediately.  The intent is
     for interactive use, but this behaviour is currently not
     implemented in 'gtroff'.  Contrary to Unix 'troff', TTY output is
     sent to a device driver also ('grotty'), making it non-trivial to
     communicate interactively.

     This request causes a line break.

 -- Request: .backtrace
     Write a backtrace of the input stack to the standard error stream.

     Consider the following in a file 'test'.

          .de xxx
          .  backtrace
          ..
          .de yyy
          .  xxx
          ..
          .
          .yyy
              error-> troff: backtrace: 'test':2: macro 'xxx'
              error-> troff: backtrace: 'test':5: macro 'yyy'
              error-> troff: backtrace: file 'test':8

     The '-b' option of GNU 'troff' causes a backtrace to be generated
     on each error or warning.  Some warnings have to be enabled; *Note
     Warnings::.

 -- Register: \n[slimit]
     If greater than 0, sets the maximum quantity of objects on GNU
     'troff''s internal input stack.  If less than or equal to 0, there
     is no limit: recursion can continue until program memory is
     exhausted.  The default is 1,000.

 -- Request: .warnscale si
     Set the scaling indicator used in warnings to SI.  Valid values for
     SI are 'u', 'i', 'c', 'p', and 'P'.  At startup, it is set to 'i'.

 -- Request: .spreadwarn [limit]
     Emit a 'break' warning if the additional space inserted for each
     space between words in an output line adjusted to both margins with
     '.ad b' is larger than or equal to LIMIT.  A negative value is
     treated as zero; an absent argument toggles the warning on and off
     without changing LIMIT.  The default scaling indicator is 'm'.  At
     startup, 'spreadwarn' is inactive and LIMIT is 3m.

     For example,

          .spreadwarn 0.2m

     causes a warning if 'break' warnings are not suppressed and
     'gtroff' must add 0.2m or more for each inter-word space in a line.
     *Note Warnings::.

   GNU 'troff' has command-line options for reporting warnings ('-w')
and backtraces ('-b') when a warning or an error occurs.

 -- Request: .warn [n]
 -- Register: \n[.warn]
     Select the categories, or "types", of reported warnings.  N is the
     sum of the numeric codes associated with each warning category that
     is to be enabled; all other categories are disabled.  The
     categories and their associated codes are listed in *note
     Warnings::.  For example, '.warn 0' disables all warnings, and
     '.warn 1' disables all warnings except those about missing glyphs.
     If no argument is given, all warning categories are enabled.

     The read-only register '.warn' contains the sum of the numeric
     codes of enabled warning categories.

5.37.1 Warnings
---------------

Warning diagnostics emitted by GNU 'troff' are divided into named,
numbered categories.  The name associated with each warning category is
used by the '-w' and '-W' options.  Each category is also assigned a
power of two; the sum of enabled category values is used by the 'warn'
request and the '.warn' register.

   Warnings of each category are produced under the following
circumstances.

'char'
'1'
     An undefined glyph was requested for output.(1)  (*note
     Warnings-Footnote-1::) This category is enabled by default.

'number'
'2'
     An invalid numeric expression was encountered.  This category is
     enabled by default.  *Note Numeric Expressions::.

'break'
'4'
     A filled output line could not be broken such that its length was
     less than the output line length '\n[.l]'.  This category is
     enabled by default.

'delim'
'8'
     The closing delimiter in an escape sequence was missing or
     mismatched.

'el'
'16'
     The 'el' request was encountered with no prior corresponding 'ie'
     request.  *Note if-else::.

'scale'
'32'
     A scaling unit inappropriate to its context was used in a numeric
     expression.

'range'
'64'
     A numeric expression was out of range for its context.

'syntax'
'128'
     A self-contradictory hyphenation mode was requested; an empty or
     incomplete numeric expression was encountered; an operand to a
     numeric operator was missing; an attempt was made to define a
     recursive, empty, or nonsensical character class; or a 'groff'
     extension conditional expression operator was used while in
     compatibility mode.

'di'
'256'
     A 'di', 'da', 'box', or 'boxa' request was invoked without an
     argument when there was no current diversion.

'mac'
'512'
     An undefined string, macro, or diversion was used.  When such an
     object is dereferenced, an empty one of that name is automatically
     created.  So, unless it is later deleted, at most one warning is
     given for each.

     This warning is also emitted upon an attempt to move an unplanted
     trap macro (*note Page Location Traps::).  In such cases, the
     unplanted macro is _not_ dereferenced, so it is not created if it
     does not exist.

'reg'
'1024'
     An undefined register was used.  When an undefined register is
     dereferenced, it is automatically defined with a value of 0.  So,
     unless it is later deleted, at most one warning is given for each.

'tab'
'2048'
     A tab character was encountered where a number was expected, or
     appeared in an unquoted macro argument.

'right-brace'
'4096'
     A right brace escape sequence '\}' was encountered where a number
     was expected.

'missing'
'8192'
     A request was invoked with a mandatory argument absent.

'input'
'16384'
     An invalid character occurred on the input stream.

'escape'
'32768'
     An unsupported escape sequence was encountered.

'space'
'65536'
     A space was missing between a request or macro and its argument.
     This warning is produced when an undefined name longer than two
     characters is encountered and the first two characters of the name
     constitute a defined name.  No request is invoked, no macro called,
     and an empty macro is not defined.  This category is enabled by
     default.  It never occurs in compatibility mode.

'font'
'131072'
     A non-existent font was selected, or the selection was ignored
     because a font selection escape sequence was used after the output
     line continuation escape sequence on an input line.  This category
     is enabled by default.

'ig'
'262144'
     An invalid escape sequence occurred in input ignored using the 'ig'
     request.  This warning category diagnoses a condition that is an
     error when it occurs in non-ignored input.

'color'
'524288'
     An undefined color was selected, an attempt was made to define a
     color using an unrecognized color space, an invalid component in a
     color definition was encountered, or an attempt was made to
     redefine a default color.

'file'
'1048576'
     An attempt was made to load a file that does not exist.  This
     category is enabled by default.

   Two warning names group other warning categories for convenience.

'all'
     All warning categories except 'di', 'mac' and 'reg'.  This
     shorthand is intended to produce all warnings that are useful with
     macro packages written for AT&T 'troff' and its descendants, which
     have less fastidious diagnostics than GNU 'troff'.

'w'
     All warning categories.  Authors of documents and macro packages
     targeting 'groff' are encouraged to use this setting.

   (1) 'char' is a misnomer since it reports missing glyphs--there are
no "missing" input characters, only invalid ones.

5.38 Implementation Differences
===============================

GNU 'troff' has a number of features that cause incompatibilities with
documents written for other versions of 'troff'.  Some GNU extensions to
'troff' have become supported by other implementations.

5.38.1 Safer Mode
-----------------

The formatter operates in "safer" mode by default; to mitigate risks
from untrusted input documents, the 'pi' and 'sy' requests are disabled.
GNU 'troff''s '-U' option enables "unsafe mode", restoring their
function and enabling additional 'groff' extension requests, 'open',
'opena', and 'pso'.  *Note I/O::.

5.38.2 Compatibility Mode
-------------------------

Long identifier names may be GNU 'troff''s most obvious innovation.
AT&T 'troff' interprets '.dsabcd' as defining a string 'ab' with
contents 'cd'.  Normally, GNU 'troff' interprets this as a call of a
macro named 'dsabcd'.  AT&T 'troff' also interprets '\*[' and '\n[' as
an interpolation of a string or register, respectively, named '['.  In
GNU 'troff', however, the '[' is normally interpreted as delimiting a
long name.  In compatibility mode, GNU 'troff' interprets names in the
traditional way; they thus can be two characters long at most.

 -- Request: .cp [n]
 -- Register: \n[.C]
     If N is missing or non-zero, turn on compatibility mode; otherwise,
     turn it off.

     The read-only register '.C' is 1 if compatibility mode is on,
     0 otherwise.

     Compatibility mode can be also turned on with the '-C' command-line
     option.

 -- Request: .do name
 -- Register: \n[.cp]
     The 'do' request interprets the string, request, diversion, or
     macro NAME (along with any further arguments) with compatibility
     mode disabled.  Compatibility mode is restored (only if it was
     active) when the _expansion_ of NAME is interpreted; that is, the
     restored compatibility state applies to the contents of the macro,
     string, or diversion NAME as well as data read from files or pipes
     if NAME is any of the 'so', 'soquiet', 'mso', 'msoquiet', or 'pso'
     requests.

     The following example illustrates several aspects of 'do' behavior.

          .de mac1
          FOO
          ..
          .de1 mac2
          groff
          .mac1
          ..
          .de mac3
          compatibility
          .mac1
          ..
          .de ma
          \\$1
          ..
          .cp 1
          .do mac1
          .do mac2 \" mac2, defined with .de1, calls "mac1"
          .do mac3 \" mac3 calls "ma" with argument "c1"
          .do mac3 \[ti] \" groff syntax accepted in .do arguments
              => FOO groff FOO compatibility c1 ~

     The read-only register '.cp', meaningful only when dereferenced
     from a 'do' request, is 1 if compatibility mode was on when the
     'do' request was encountered, and 0 if it was not.  This register
     is specialized and may require a statement of rationale.

     When writing macro packages or documents that use GNU 'troff'
     features and which may be mixed with other packages or documents
     that do not--common scenarios include serial processing of man
     pages or use of the 'so' or 'mso' requests--you may desire correct
     operation regardless of compatibility mode enablement in the
     surrounding context.  It may occur to you to save the existing
     value of '\n(.C' into a register, say, '_C', at the beginning of
     your file, turn compatibility mode off with '.cp 0', then restore
     it from that register at the end with '.cp \n(_C'.  At the same
     time, a modular design of a document or macro package may lead you
     to multiple layers of inclusion.  You cannot use the same register
     name everywhere lest you "clobber" the value from a preceding or
     enclosing context.  The two-character register name space of AT&T
     'troff' is confining and mnemonically challenging; you may wish to
     use the more capacious name space of GNU 'troff'.  However,
     attempting '.nr _my_saved_C \n(.C' will not work in compatibility
     mode; the register name is too long.  "This is exactly what 'do' is
     for," you think, '.do nr _my_saved_C \n(.C'.  The foregoing will
     always save zero to your register, because 'do' turns compatibility
     mode _off_ while it interprets its argument list.

     To robustly save compatibility mode before switching it off, use

          .do nr _my_saved_C \n[.cp]
          .cp 0

     at the beginning of your file, followed by

          .cp \n[_my_saved_C]
          .do rr _my_saved_C

     at the end.  As in the C language, we all have to share one big
     name space, so choose a register name that is unlikely to collide
     with other uses.

   Normally, GNU 'troff' preserves the interpolation depth in delimited
arguments, but not in compatibility mode.

     .ds xx '
     \w'abc\*(xxdef'
         => 168 (normal mode on a terminal device)
         => 72def' (compatibility mode on a terminal device)

   Furthermore, the escape sequences '\f', '\H', '\m', '\M', '\R', '\s',
and '\S' are transparent for the purpose of recognizing a control
character at the beginning of a line only in compatibility mode.  For
example, this code produces bold output in both cases, but the text
differs.

     .de xx
     Hello!
     ..
     \fB.xx\fP
         => .xx (normal mode)
         => Hello! (compatibility mode)

   Normally, the syntax form '\s'N accepts only a single character (a
digit) for N, consistently with other forms that originated in AT&T
'troff', like '\*', '\$', '\f', '\g', '\k', '\n', and '\z'.  In
compatibility mode only, a non-zero N must be in the range 4-39.  Legacy
documents relying upon this quirk of parsing(1) (*note Compatibility
Mode-Footnote-1::) should be migrated to another '\s' form.

   (1) The Graphic Systems C/A/T phototypesetter (the original device
target for AT&T 'troff') supported only a few discrete type sizes in the
range 6-36 points, so Ossanna contrived a special case in the parser to
do what the user must have meant.  Kernighan warned of this in the 1992
revision of CSTR #54 (2.3), and more recently, McIlroy referred to it
as a "living fossil".

5.38.3 Other Differences
------------------------

'groff' request names unrecognized by other 'troff' implementations will
likely be ignored by them; escape sequences that are 'groff' extensions
are liable to be interpreted as if the escape character were not
present.  For example, the adjustable, non-breaking escape sequence '\~'
is also supported by Heirloom Doctools 'troff' 050915 (September 2005),
'mandoc' 1.9.5 (2009-09-21), 'neatroff' (commit 1c6ab0f6e, 2016-09-13),
and Plan 9 from User Space 'troff' (commit 93f8143600, 2022-08-12), but
not by Solaris or Documenter's Workbench 'troff's.  *Note Manipulating
Filling and Adjustment::.

   GNU 'troff' does not allow the use of the escape sequences '\|',
'\^', '\&', '\{', '\}', '\<SP>', '\'', '\`', '\-', '\_', '\!', '\%', and
'\c' in identifiers; AT&T 'troff' does.  The '\A' escape sequence (*note
Identifiers::) may be helpful in avoiding use of these escape sequences
in names.

   When adjusting to both margins, AT&T 'troff' at first adjusts spaces
starting from the right; GNU 'troff' begins from the left.  Both
implementations adjust spaces from opposite ends on alternating output
lines in this adjustment mode to prevent "rivers" in the text.

   GNU 'troff' does not always hyphenate words as AT&T 'troff' does.
The AT&T implementation uses a set of hard-coded rules specific to
English, while GNU 'troff' uses language-specific hyphenation pattern
files derived from TeX.  Furthermore, in old versions of 'troff' there
was a limited amount of space to store hyphenation exceptions (arguments
to the 'hw' request); GNU 'troff' has no such restriction.

   GNU 'troff' predefines a string '.T' containing the argument given to
the '-T' command-line option, namely the current output device (for
example, 'pdf' or 'utf8').  The existence of this string is a common
feature of post-CSTR #54 'troff's(1) (*note Other
Differences-Footnote-1::) but valid values are specific to each
implementation.

   The (read-only) register '.T' interpolates 1 if GNU 'troff' is called
with the '-T' command-line option, and 0 otherwise.  This behavior
differs from AT&T 'troff', which interpolated 1 only if 'nroff' was the
formatter and was called with '-T'.

   AT&T 'troff' and other implementations handle the 'lf' request
differently.  For them, its LINE argument changes the line number of the
_current_ line.

   AT&T 'troff' had only environments named '0', '1', and '2'.  In GNU
'troff', any number of environments may exist, using any valid
identifiers for their names (*note Identifiers::.)

   Fractional type sizes cause one noteworthy incompatibility.  In AT&T
'troff' the 'ps' request ignores scale indicators and thus '.ps 10u'
sets the type size to 10 points, whereas in GNU 'troff' it sets the type
size to 10 _scaled_ points.  *Note Using Fractional Type Sizes::.

   The 'ab' request differs from AT&T 'troff': GNU 'troff' writes no
message to the standard error stream if no arguments are given, and it
exits with a failure status instead of a successful one.

   The 'bp' request differs from AT&T 'troff': GNU 'troff' does not
accept a scaling indicator on the argument, a page number; the former
(somewhat uselessly) does.

   The 'pm' request differs from AT&T 'troff': GNU 'troff' reports the
sizes of macros, strings, and diversions in bytes and ignores an
argument to report only the sum of the sizes.

   Unlike AT&T 'troff', GNU 'troff' does not ignore the 'ss' request if
the output is a terminal device; instead, the values of minimal
inter-word and additional inter-sentence space are each rounded down to
the nearest multiple of 12.

   In GNU 'troff' there is a fundamental difference between
(unformatted) input characters and (formatted) output glyphs.
Everything that affects how a glyph is output is stored with the glyph
node; once a glyph node has been constructed, it is unaffected by any
subsequent requests that are executed, including 'bd', 'cs', 'tkf',
'tr', or 'fp' requests.  Normally, glyphs are constructed from input
characters immediately before the glyph is added to the current output
line.  Macros, diversions, and strings are all, in fact, the same type
of object; they contain lists of input characters and glyph nodes in any
combination.  Special characters can be both: before being added to the
output, they act as input entities; afterward, they denote glyphs.  A
glyph node does not behave like an input character for the purposes of
macro processing; it does not inherit any of the special properties that
the input character from which it was constructed might have had.
Consider the following example.

     .di x
     \\\\
     .br
     .di
     .x

It prints '\\' in GNU 'troff'; each pair of input backslashes is turned
into one output backslash and the resulting output backslashes are not
interpreted as escape characters when they are reread.  AT&T 'troff'
would interpret them as escape characters when they were reread and
would end up printing one '\'.

   One correct way to obtain a printable backslash in most documents is
to use the '\e' escape sequence; this always prints a single instance of
the current escape character,(2) (*note Other Differences-Footnote-2::)
regardless of whether or not it is used in a diversion; it also works in
both GNU 'troff' and AT&T 'troff'.

   The other correct way, appropriate in contexts independent of the
backslash's common use as a 'troff' escape character--perhaps in
discussion of character sets or other programming languages--is the
character escape '\(rs' or '\[rs]', for "reverse solidus", from its name
in the ECMA-6 (ISO/IEC 646) standard.(3)  (*note Other
Differences-Footnote-3::)

   To store an escape sequence in a diversion that is interpreted when
the diversion is reread, either use the traditional '\!' transparent
output facility, or, if this is unsuitable, the new '\?' escape
sequence.  *Note Diversions:: and *note Gtroff Internals::.

   In the somewhat pathological case where a diversion exists containing
a partially collected line and a partially collected line at the
top-level diversion has never existed, AT&T 'troff' will output the
partially collected line at the end of input; GNU 'troff' will not.

   (1) DWB 3.3, Solaris, Heirloom Doctools, and Plan 9 'troff' all
support it.

   (2) Naturally, if you've changed the escape character, you need to
prefix the 'e' with whatever it is--and you'll likely get something
other than a backslash in the output.

   (3) The 'rs' special character identifier was not defined in AT&T
'troff''s font description files, but is in those if its lineal
descendant, Heirloom Doctools 'troff', as of the latter's 060716 release
(July 2006).

6 File Formats
**************

All files read and written by 'gtroff' are text files.  The following
two sections describe their format.

6.1 'gtroff' Output
===================

This section describes the 'groff' intermediate output format produced
by GNU 'troff'.

   As 'groff' is a wrapper program around GNU 'troff' and automatically
calls an output driver (or "postprocessor"), this output does not show
up normally.  This is why it is called _intermediate_.  'groff' provides
the option '-Z' to inhibit postprocessing such that the produced
intermediate output is sent to standard output just as it is when
calling GNU 'troff' directly.

   Here, the term "troff output" describes what is output by GNU
'troff', while "intermediate output" refers to the language that is
accepted by the parser that prepares this output for the output drivers.
This parser handles whitespace more flexibly than AT&T's implementation
and implements obsolete elements for compatibility; otherwise, both
formats are the same.(1)  (*note gtroff Output-Footnote-1::)

   The main purpose of the intermediate output concept is to facilitate
the development of postprocessors by providing a common programming
interface for all devices.  It has a language of its own that is
completely different from the 'gtroff' language.  While the 'gtroff'
language is a high-level programming language for text processing, the
intermediate output language is a kind of low-level assembler language
by specifying all positions on the page for writing and drawing.

   The intermediate output produced by 'gtroff' is fairly readable,
while output from AT&T 'troff' is rather hard to understand because of
strange habits that are still supported, but not used any longer by
'gtroff'.

   (1) The parser and postprocessor for intermediate output can be found
in the file
'GROFF-SOURCE-DIR/src/libs/libdriver/input.cpp'.

6.1.1 Language Concepts
-----------------------

The fundamental operation of the GNU 'troff' formatter is the
translation of the 'groff' input language into a device-independent form
primarily concerned with what has to be written or drawn at specific
positions on the output device.  This language is simple and imperative.
In the following discussion, the term "command" always refers to this
intermediate output language, and never to the 'groff' language intended
for direct use by document authors.  Intermediate output commands
comprise several categories: glyph output; font, color, and text size
selection; motion of the printing position; page advancement; drawing of
geometric primitives; and device control commands, a catch-all for
operations not easily classified as any of the foregoing, such as
directives to start and stop output, identify the intended output
device, or place URL hyperlinks in supported output formats.

6.1.1.1 Separation
..................

AT&T 'troff' output has strange requirements regarding whitespace.  The
'gtroff' output parser, however, is more tolerant, making whitespace
maximally optional.  Such characters, i.e., the tab, space, and newline,
always have a syntactical meaning.  They are never printable because
spacing within the output is always done by positioning commands.

   Any sequence of space or tab characters is treated as a single
"syntactical space".  It separates commands and arguments, but is only
required when there would occur a clashing between the command code and
the arguments without the space.  Most often, this happens when
variable-length command names, arguments, argument lists, or command
clusters meet.  Commands and arguments with a known, fixed length need
not be separated by syntactical space.

   A line break is a syntactical element, too.  Every command argument
can be followed by whitespace, a comment, or a newline character.  Thus
a "syntactical line break" is defined to consist of optional syntactical
space that is optionally followed by a comment, and a newline character.

   The normal commands, those for positioning and text, consist of a
single letter taking a fixed number of arguments.  For historical
reasons, the parser allows stacking of such commands on the same line,
but fortunately, in 'gtroff''s intermediate output, every command with
at least one argument is followed by a line break, thus providing
excellent readability.

   The other commands--those for drawing and device controlling--have a
more complicated structure; some recognize long command names, and some
take a variable number of arguments.  So all 'D' and 'x' commands were
designed to request a syntactical line break after their last argument.
Only one command, 'x X', has an argument that can span several input
lines; all other commands must have all of their arguments on the same
line as the command, i.e., the arguments may not be split by a line
break.

   Empty lines (these are lines containing only space and/or a comment),
can occur everywhere.  They are just ignored.

6.1.1.2 Argument Units
......................

Some commands take integer arguments that are assumed to represent
values in a measurement unit, but the letter for the corresponding scale
indicator is not written with the output command arguments.  Most
commands assume the scaling indicator 'u', the basic unit of the device,
some use 'z', the scaled point unit of the device, while others, such as
the color commands, expect plain integers.

   Single characters can have the eighth bit set, as can the names of
fonts and special characters.  The names of characters and fonts can be
of arbitrary length.  A character that is to be printed is always in the
current font.

   A string argument is always terminated by the next whitespace
character (space, tab, or newline); an embedded '#' character is
regarded as part of the argument, not as the beginning of a comment
command.  An integer argument is already terminated by the next
non-digit character, which then is regarded as the first character of
the next argument or command.

6.1.1.3 Document Parts
......................

A correct intermediate output document consists of two parts, the
"prologue" and the "body".

   The task of the prologue is to set the general device parameters
using three exactly specified commands.  'gtroff''s prologue is
guaranteed to consist of the following three lines (in that order):

     x T DEVICE
     x res N H V
     x init

with the arguments set as outlined in *note Device Control Commands::.
The parser for the intermediate output format is able to interpret
additional whitespace and comments as well even in the prologue.

   The body is the main section for processing the document data.
Syntactically, it is a sequence of any commands different from the ones
used in the prologue.  Processing is terminated as soon as the first
'x stop' command is encountered; the last line of any 'gtroff'
intermediate output always contains such a command.

   Semantically, the body is page oriented.  A new page is started by a
'p' command.  Positioning, writing, and drawing commands are always done
within the current page, so they cannot occur before the first 'p'
command.  Absolute positioning (by the 'H' and 'V' commands) is done
relative to the current page; all other positioning is done relative to
the current location within this page.

6.1.2 Command Reference
-----------------------

This section describes all intermediate output commands, both from AT&T
'troff' as well as the 'gtroff' extensions.

6.1.2.1 Comment Command
.......................

'#ANYTHING<end of line>'
     A comment.  Ignore any characters from the '#' character up to the
     next newline character.

     This command is the only possibility for commenting in the
     intermediate output.  Each comment can be preceded by arbitrary
     syntactical space; every command can be terminated by a comment.

6.1.2.2 Simple Commands
.......................

The commands in this subsection have a command code consisting of a
single character, taking a fixed number of arguments.  Most of them are
commands for positioning and text writing.  These commands are tolerant
of whitespace.  Optionally, syntactical space can be inserted before,
after, and between the command letter and its arguments.  All of these
commands are stackable; i.e., they can be preceded by other simple
commands or followed by arbitrary other commands on the same line.  A
separating syntactical space is only necessary when two integer
arguments would clash or if the preceding argument ends with a string
argument.

'C XXX<whitespace>'
     Print a special character named XXX.  The trailing syntactical
     space or line break is necessary to allow glyph names of arbitrary
     length.  The glyph is printed at the current print position; the
     glyph's size is read from the font file.  The print position is not
     changed.

'c G'
     Print glyph G at the current print position;(1) (*note Simple
     Commands-Footnote-1::) the glyph's size is read from the font file.
     The print position is not changed.

'f N'
     Set font to font number N (a non-negative integer).

'H N'
     Move right to the absolute vertical position N (a non-negative
     integer in basic units 'u' relative to left edge of current page.

'h N'
     Move N (a non-negative integer) basic units 'u' horizontally to the
     right.  The AT&T 'troff' manual allows negative values for N also,
     but GNU 'troff' doesn't use them.

'm COLOR-SCHEME [COMPONENT ...]'
     Set the color for text (glyphs), line drawing, and the outline of
     graphic objects using different color schemes; the analogous
     command for the filling color of graphic objects is 'DF'.  The
     color components are specified as integer arguments between 0 and
     65535.  The number of color components and their meaning vary for
     the different color schemes.  These commands are generated by
     'gtroff''s escape sequence '\m'.  No position changing.  These
     commands are a 'gtroff' extension.

     'mc CYAN MAGENTA YELLOW'
          Set color using the CMY color scheme, having the 3 color
          components CYAN, MAGENTA, and YELLOW.

     'md'
          Set color to the default color value (black in most cases).
          No component arguments.

     'mg GRAY'
          Set color to the shade of gray given by the argument, an
          integer between 0 (black) and 65535 (white).

     'mk CYAN MAGENTA YELLOW BLACK'
          Set color using the CMYK color scheme, having the 4 color
          components CYAN, MAGENTA, YELLOW, and BLACK.

     'mr RED GREEN BLUE'
          Set color using the RGB color scheme, having the 3 color
          components RED, GREEN, and BLUE.

'N N'
     Print glyph with index N (a non-negative integer) of the current
     font.  This command is a 'gtroff' extension.

'n B A'
     Inform the device about a line break, but no positioning is done by
     this command.  In AT&T 'troff', the integer arguments B and A
     informed about the space before and after the current line to make
     the intermediate output more human readable without performing any
     action.  In 'groff', they are just ignored, but they must be
     provided for compatibility reasons.

'p N'
     Begin a new page in the output.  The page number is set to N.  This
     page is completely independent of pages formerly processed even if
     those have the same page number.  The vertical position on the
     output is automatically set to 0.  All positioning, writing, and
     drawing is always done relative to a page, so a 'p' command must be
     issued before any of these commands.

's N'
     Set type size to N scaled points (this is unit 'z').  AT&T 'troff'
     used the unit points ('p') instead.  *Note Output Language
     Compatibility::.

't XXX<whitespace>'
't XXX DUMMY-ARG<whitespace>'
     Print a word, i.e., a sequence of characters XXX representing
     output glyphs which names are single characters, terminated by a
     space character or a line break; an optional second integer
     argument is ignored (this allows the formatter to generate an even
     number of arguments).  The first glyph should be printed at the
     current position, the current horizontal position should then be
     increased by the width of the first glyph, and so on for each
     glyph.  The widths of the glyphs are read from the font file,
     scaled for the current type size, and rounded to a multiple of the
     horizontal motion quantum.  Special characters cannot be printed
     using this command (use the 'C' command for special characters).
     This command is a 'gtroff' extension; it is only used for devices
     whose 'DESC' file contains the 'tcommand' keyword (*note DESC File
     Format::).

'u N XXX<whitespace>'
     Print word with track kerning.  This is the same as the 't' command
     except that after printing each glyph, the current horizontal
     position is increased by the sum of the width of that glyph and N
     (an integer in basic units 'u').  This command is a 'gtroff'
     extension; it is only used for devices whose 'DESC' file contains
     the 'tcommand' keyword (*note DESC File Format::).

'V N'
     Move down to the absolute vertical position N (a non-negative
     integer in basic units 'u') relative to upper edge of current page.

'v N'
     Move N basic units 'u' down (N is a non-negative integer).  The
     AT&T 'troff' manual allows negative values for N also, but GNU
     'troff' doesn't use them.

'w'
     Describe an adjustable space.  This performs no action; it is
     present for documentary purposes.  The spacing itself must be
     performed explicitly by a move command.

   (1) 'c' is actually a misnomer since it outputs a glyph.

6.1.2.3 Graphics Commands
.........................

Each graphics or drawing command in the intermediate output starts with
the letter 'D', followed by one or two characters that specify a
subcommand; this is followed by a fixed or variable number of integer
arguments that are separated by a single space character.  A 'D' command
may not be followed by another command on the same line (apart from a
comment), so each 'D' command is terminated by a syntactical line break.

   'gtroff' output follows the classical spacing rules (no space between
command and subcommand, all arguments are preceded by a single space
character), but the parser allows optional space between the command
letters and makes the space before the first argument optional.  As
usual, each space can be any sequence of tab and space characters.

   Some graphics commands can take a variable number of arguments.  In
this case, they are integers representing a size measured in basic units
'u'.  The arguments called H1, H2, ..., HN stand for horizontal
distances where positive means right, negative left.  The arguments
called V1, V2, ..., VN stand for vertical distances where positive means
down, negative up.  All these distances are offsets relative to the
current location.

   Each graphics command directly corresponds to a similar 'gtroff' '\D'
escape sequence.  *Note Drawing Requests::.

   Unknown 'D' commands are assumed to be device-specific.  Its
arguments are parsed as strings; the whole information is then sent to
the postprocessor.

   In the following command reference, the syntax element <line break>
means a syntactical line break as defined above.

'D~ H1 V1 H2 V2 ... HN VN<line break>'
     Draw B-spline from current position to offset (H1,V1), then to
     offset (H2,V2), if given, etc., up to (HN,VN).  This command takes
     a variable number of argument pairs; the current position is moved
     to the terminal point of the drawn curve.

'Da H1 V1 H2 V2<line break>'
     Draw arc from current position to (H1,V1)+(H2,V2) with center at
     (H1,V1); then move the current position to the final point of the
     arc.

'DC D<line break>'
'DC D DUMMY-ARG<line break>'
     Draw a solid circle using the current fill color with diameter D
     (integer in basic units 'u') with leftmost point at the current
     position; then move the current position to the rightmost point of
     the circle.  An optional second integer argument is ignored (this
     allows the formatter to generate an even number of arguments).
     This command is a 'gtroff' extension.

'Dc D<line break>'
     Draw circle line with diameter D (integer in basic units 'u') with
     leftmost point at the current position; then move the current
     position to the rightmost point of the circle.

'DE H V<line break>'
     Draw a solid ellipse in the current fill color with a horizontal
     diameter of H and a vertical diameter of V (both integers in basic
     units 'u') with the leftmost point at the current position; then
     move to the rightmost point of the ellipse.  This command is a
     'gtroff' extension.

'De H V<line break>'
     Draw an outlined ellipse with a horizontal diameter of H and a
     vertical diameter of V (both integers in basic units 'u') with the
     leftmost point at current position; then move to the rightmost
     point of the ellipse.

'DF COLOR-SCHEME [COMPONENT ...]<line break>'
     Set fill color for solid drawing objects using different color
     schemes; the analogous command for setting the color of text, line
     graphics, and the outline of graphic objects is 'm'.  The color
     components are specified as integer arguments between 0 and 65535.
     The number of color components and their meaning vary for the
     different color schemes.  These commands are generated by
     'gtroff''s escape sequences '\D'F ...'' and '\M' (with no other
     corresponding graphics commands).  No position changing.  This
     command is a 'gtroff' extension.

     'DFc CYAN MAGENTA YELLOW<line break>'
          Set fill color for solid drawing objects using the CMY color
          scheme, having the 3 color components CYAN, MAGENTA, and
          YELLOW.

     'DFd<line break>'
          Set fill color for solid drawing objects to the default fill
          color value (black in most cases).  No component arguments.

     'DFg GRAY<line break>'
          Set fill color for solid drawing objects to the shade of gray
          given by the argument, an integer between 0 (black) and 65535
          (white).

     'DFk CYAN MAGENTA YELLOW BLACK<line break>'
          Set fill color for solid drawing objects using the CMYK color
          scheme, having the 4 color components CYAN, MAGENTA, YELLOW,
          and BLACK.

     'DFr RED GREEN BLUE<line break>'
          Set fill color for solid drawing objects using the RGB color
          scheme, having the 3 color components RED, GREEN, and BLUE.

'Df N<line break>'
     The argument N must be an integer in the range -32767 to 32767.

     0 <= N <= 1000
          Set the color for filling solid drawing objects to a shade of
          gray, where 0 corresponds to solid white, 1000 (the default)
          to solid black, and values in between to intermediate shades
          of gray; this is obsoleted by command 'DFg'.

     N < 0 or N > 1000
          Set the filling color to the color that is currently being
          used for the text and the outline, see command 'm'.  For
          example, the command sequence

               mg 0 0 65535
               Df -1

          sets all colors to blue.

     No position changing.  This command is a 'gtroff' extension.

'Dl H V<line break>'
     Draw line from current position to offset (H,V) (integers in basic
     units 'u'); then set current position to the end of the drawn line.

'Dp H1 V1 H2 V2 ... HN VN<line break>'
     Draw a polygon line from current position to offset (H1,V1), from
     there to offset (H2,V2), etc., up to offset (HN,VN), and from there
     back to the starting position.  For historical reasons, the
     position is changed by adding the sum of all arguments with odd
     index to the actual horizontal position and the even ones to the
     vertical position.  Although this doesn't make sense it is kept for
     compatibility.  This command is a 'gtroff' extension.

'DP H1 V1 H2 V2 ... HN VN<line break>'
     Draw a solid polygon in the current fill color rather than an
     outlined polygon, using the same arguments and positioning as the
     corresponding 'Dp' command.  This command is a 'gtroff' extension.

'Dt N<line break>'
     Set the current line thickness to N (an integer in basic units 'u')
     if N>0; if N=0 select the smallest available line thickness; if N<0
     set the line thickness proportional to the type size (this is the
     default before the first 'Dt' command was specified).  For
     historical reasons, the horizontal position is changed by adding
     the argument to the actual horizontal position, while the vertical
     position is not changed.  Although this doesn't make sense it is
     kept for compatibility.  This command is a 'gtroff' extension.

6.1.2.4 Device Control Commands
...............................

Each device control command starts with the letter 'x', followed by a
space character (optional or arbitrary space or tab in 'gtroff') and a
subcommand letter or word; each argument (if any) must be preceded by a
syntactical space.  All 'x' commands are terminated by a syntactical
line break; no device control command can be followed by another command
on the same line (except a comment).

   The subcommand is basically a single letter, but to increase
readability, it can be written as a word, i.e., an arbitrary sequence of
characters terminated by the next tab, space, or newline character.  All
characters of the subcommand word but the first are simply ignored.  For
example, 'gtroff' outputs the initialization command 'x i' as 'x init'
and the resolution command 'x r' as 'x res'.

   In the following, the syntax element <line break> means a syntactical
line break (*note Separation::).

'xF NAME<line break>'
     The 'F' stands for FILENAME.

     Use NAME as the intended name for the current file in error
     reports.  This is useful for remembering the original file name
     when 'gtroff' uses an internal piping mechanism.  The input file is
     not changed by this command.  This command is a 'gtroff' extension.

'xf N S<line break>'
     The 'f' stands for FONT.

     Mount font position N (a non-negative integer) with font named S (a
     text word).  *Note Font Positions::.

'xH N<line break>'
     The 'H' stands for HEIGHT.

     Set glyph height to N (a positive integer in scaled points 'z').
     AT&T 'troff' uses the unit points ('p') instead.  *Note Output
     Language Compatibility::.

'xi<line break>'
     The 'i' stands for INIT.

     Initialize device.  This is the third command of the prologue.

'xp<line break>'
     The 'p' stands for PAUSE.

     Parsed but ignored.  The AT&T 'troff' manual documents this command
     as

          pause device, can be restarted

     but GNU 'troff' output drivers do nothing with this command.

'xr N H V<line break>'
     The 'r' stands for RESOLUTION.

     Resolution is N, while H is the minimal horizontal motion, and V
     the minimal vertical motion possible with this device; all
     arguments are positive integers in basic units 'u' per inch.  This
     is the second command of the prologue.

'xS N<line break>'
     The 'S' stands for SLANT.

     Set slant to N (an integer in basic units 'u').

'xs<line break>'
     The 's' stands for STOP.

     Terminates the processing of the current file; issued as the last
     command of any intermediate 'troff' output.

'xt<line break>'
     The 't' stands for TRAILER.

     Generate trailer information, if any.  In GNU 'troff', this is
     ignored.

'xT XXX<line break>'
     The 'T' stands for TYPESETTER.

     Set the name of the output driver to XXX, a sequence of
     non-whitespace characters terminated by whitespace.  The possible
     names correspond to those of 'groff''s '-T' option.  This is the
     first command of the prologue.

'xu N<line break>'
     The 'u' stands for UNDERLINE.

     Configure underlining of spaces.  If N is 1, start underlining of
     spaces; if N is 0, stop underlining of spaces.  This is needed for
     the 'cu' request in 'nroff' mode and is ignored otherwise.  This
     command is a 'gtroff' extension.

'xX ANYTHING<line break>'
     The 'x' stands for X-ESCAPE.

     Send string ANYTHING uninterpreted to the device.  If the line
     following this command starts with a '+' character this line is
     interpreted as a continuation line in the following sense.  The '+'
     is ignored, but a newline character is sent instead to the device,
     the rest of the line is sent uninterpreted.  The same applies to
     all following lines until the first character of a line is not a
     '+' character.  This command is generated by the 'gtroff' escape
     sequence '\X'.  The line-continuing feature is a 'gtroff'
     extension.

6.1.2.5 Obsolete Command
........................

In AT&T 'troff' output, the writing of a single glyph is mostly done by
a very strange command that combines a horizontal move and a single
character giving the glyph name.  It doesn't have a command code, but is
represented by a 3-character argument consisting of exactly 2 digits and
a character.

DDG
     Move right DD (exactly two decimal digits) basic units 'u', then
     print glyph G (represented as a single character).

     In GNU 'troff', arbitrary syntactical space around and within this
     command is allowed.  Only when a preceding command on the same line
     ends with an argument of variable length is a separating space
     obligatory.  In AT&T 'troff', large clusters of these and other
     commands are used, mostly without spaces; this made such output
     almost unreadable.

   For modern high-resolution devices, this command does not make sense
because the width of the glyphs can become much larger than two decimal
digits.  In 'gtroff', this is only used for the devices 'X75', 'X75-12',
'X100', and 'X100-12'.  For other devices, the commands 't' and 'u'
provide a better functionality.

6.1.3 Intermediate Output Examples
----------------------------------

This section presents the intermediate output generated from the same
input for three different devices.  The input is the sentence 'hell
world' fed into 'gtroff' on the command line.

High-resolution device 'ps'

     This is the standard output of 'gtroff' if no '-T' option is given.

          shell> echo "hell world" | groff -Z -T ps

          x T ps
          x res 72000 1 1
          x init
          p1
          x font 5 TR
          f5
          s10000
          V12000
          H72000
          thell
          wh2500
          tw
          H96620
          torld
          n12000 0
          x trailer
          V792000
          x stop

     This output can be fed into 'grops' to get its representation as a
     PostScript file.

Low-resolution device 'latin1'

     This is similar to the high-resolution device except that the
     positioning is done at a minor scale.  Some comments (lines
     starting with '#') were added for clarification; they were not
     generated by the formatter.

          shell> echo "hell world" | groff -Z -T latin1

          # prologue
          x T latin1
          x res 240 24 40
          x init
          # begin a new page
          p1
          # font setup
          x font 1 R
          f1
          s10
          # initial positioning on the page
          V40
          H0
          # write text 'hell'
          thell
          # inform about space, and issue a horizontal jump
          wh24
          # write text 'world'
          tworld
          # announce line break, but do nothing because...
          n40 0
          # ...the end of the document has been reached
          x trailer
          V2640
          x stop

     This output can be fed into 'grotty' to get a formatted text
     document.

AT&T 'troff' output
     Since a computer monitor has a much lower resolution than modern
     printers, the intermediate output for X11 devices can use the
     jump-and-write command with its 2-digit displacements.

          shell> echo "hell world" | groff -Z -T X100

          x T X100
          x res 100 1 1
          x init
          p1
          x font 5 TR
          f5
          s10
          V16
          H100
          # write text with jump-and-write commands
          ch07e07l03lw06w11o07r05l03dh7
          n16 0
          x trailer
          V1100
          x stop

     This output can be fed into 'xditview' or 'gxditview' for
     displaying in X.

     Due to the obsolete jump-and-write command, the text clusters in
     the AT&T 'troff' output are almost unreadable.

6.1.4 Output Language Compatibility
-----------------------------------

The intermediate output language of AT&T 'troff' was first documented in
'A Typesetter-independent TROFF', by Brian Kernighan, and by 1992 the
AT&T 'troff' manual was updated to incorprate a description of it.

   The GNU 'troff' intermediate output format is compatible with this
specification except for the following features.

   * The classical quasi-device independence is not yet implemented.

   * The old hardware was very different from what we use today.  So the
     'groff' devices are also fundamentally different from the ones in
     AT&T 'troff'.  For example, the AT&T PostScript device is called
     'post' and has a resolution of only 720 units per inch, suitable
     for printers 20 years ago, while 'groff''s 'ps' device has a
     resolution of 72000 units per inch.  Maybe, by implementing some
     rescaling mechanism similar to the classical quasi-device
     independence, 'groff' could emulate AT&T's 'post' device.

   * The B-spline command 'D~' is correctly handled by the intermediate
     output parser, but the drawing routines aren't implemented in some
     of the postprocessor programs.

   * The argument of the commands 's' and 'x H' has the implicit unit
     scaled point 'z' in 'gtroff', while AT&T 'troff' has point ('p').
     This isn't an incompatibility but a compatible extension, for both
     units coincide for all devices without a 'sizescale' parameter in
     the 'DESC' file, including all postprocessors from AT&T and
     'groff''s text devices.  The few 'groff' devices with a 'sizescale'
     parameter either do not exist for AT&T 'troff', have a different
     name, or seem to have a different resolution.  So conflicts are
     very unlikely.

   * The position changing after the commands 'Dp', 'DP', and 'Dt' is
     illogical, but as old versions of 'gtroff' used this feature it is
     kept for compatibility reasons.

6.2 Device and Font Description Files
=====================================

The 'groff' font and output device description formats are slight
extensions of those used by AT&T device-independent 'troff'.  In
distinction to the AT&T implementation, 'groff' lacks a binary format;
all files are text files.(1)  (*note Device and Font Description
Files-Footnote-1::) The device and font description files for a device
NAME are stored in a 'devNAME' directory.  The device description file
is called 'DESC', and, for each font supported by the device, a font
description file is called 'F', where F is usually an abbreviation of a
font's name and/or style.  For example, the 'ps' (PostScript) device has
'groff' font description files for Times roman ('TR') and Zapf Chancery
Medium italic ('ZCMI'), among many others, while the 'utf8' device (for
terminal emulators) has only font descriptions for the roman, italic,
bold, and bold-italic styles ('R', 'I', 'B', and 'BI', respectively).

   Device and font description files are read both by the formatter, GNU
'troff', and by output drivers.  The programs delegate these files'
processing to an internal library, 'libgroff', ensuring their consistent
interpretation.

   (1) Plan 9 'troff' has also abandoned the binary format.

6.2.1 'DESC' File Format
------------------------

The 'DESC' file contains a series of directives; each begins a line.
Their order is not important, with two exceptions: (1) the 'res'
directive must precede any 'papersize' directive; and (2) the 'charset'
directive must come last (if at all).  If a directive name is repeated,
later entries in the file override previous ones (except that the paper
dimensions are computed based on the 'res' directive last seen when
'papersize' is encountered).  Spaces and/or tabs separate words and are
ignored at line boundaries.  Comments start with the '#' character and
extend to the end of a line.  Empty lines are ignored.

'family FAM'
     The default font family is FAM.

'fonts N F1 ... FN'
     Fonts F1, ..., FN are mounted at font positions M+1, ..., M+N where
     M is the number of 'styles' (see below).  This directive may extend
     over more than one line.  A font name of '0' causes no font to be
     mounted at the corresponding position.

'hor N'
     The horizontal motion quantum is N basic units.  All horizontal
     quantities are rounded to multiples of N.

'image_generator PROGRAM'
     Use PROGRAM to generate PNG images from PostScript input.  Under
     GNU/Linux, this is usually 'gs', but under other systems (notably
     Cygwin) it might be set to another name.  The 'grohtml' driver uses
     this directive.

'paperlength N'
     The vertical dimension of the output medium is N basic units
     (deprecated: use 'papersize' instead).

'papersize FORMAT-OR-DIMENSION-PAIR-OR-FILE-NAME ...'
     The dimensions of the output medium are as according to the
     argument, which is either a standard paper format, a pair of
     dimensions, or the name of a plain text file containing either of
     the foregoing.

     Recognized paper formats are the ISO and DIN formats 'A0'-'A7',
     'B0'-'B7', 'C0'-'C7', 'D0'-'D7'; the U.S. paper types 'letter',
     'legal', 'tabloid', 'ledger', 'statement', and 'executive'; and the
     envelope formats 'com10', 'monarch', and 'DL'.  Matching is
     performed without regard for lettercase.

     Alternatively, the argument can be a custom paper format in the
     format 'LENGTH,WIDTH' (with no spaces before or after the comma).
     Both LENGTH and WIDTH must have a unit appended; valid units are
     'i' for inches, 'c' for centimeters, 'p' for points, and 'P' for
     picas.  Example: '12c,235p'.  An argument that starts with a digit
     is always treated as a custom paper format.

     Finally, the argument can be a file name (e.g., '/etc/papersize');
     if the file can be opened, the first line is read and a match
     attempted against each of the other forms.  No comment syntax is
     supported.

     More than one argument can be specified; each is scanned in turn
     and the first valid paper specification used.

'paperwidth N'
     The horizontal dimension of the output medium is N basic units
     (deprecated: use 'papersize' instead).

'pass_filenames'
     Direct GNU 'troff' to emit the name of the source file being
     processed.  This is achieved with the intermediate output command
     'x F', which 'grohtml' interprets.

'postpro PROGRAM'
     Use PROGRAM as the postprocessor.

'prepro PROGRAM'
     Use PROGRAM as a preprocessor.  The 'html' and 'xhtml' output
     devices use this directive.

'print PROGRAM'
     Use PROGRAM as a spooler program for printing.  If omitted, the
     '-l' and '-L' options of 'groff' are ignored.

'res N'
     The device resolution is N basic units per inch.

'sizes S1 ... SN 0'
     The device has fonts at S1, ..., SN scaled points (see below).  The
     list of sizes must be terminated by '0'.  Each SI can also be a
     range of sizes M-N.  The list can extend over more than one line.

'sizescale N'
     A typographical point is subdivided into N scaled points.  The
     default is '1'.  *Note Using Fractional Type Sizes::.

'styles S1 ... SM'
     The first M font mounting positions are associated with styles S1,
     ..., SM.

'tcommand'
     The postprocessor can handle the 't' and 'u' intermediate output
     commands.

'unicode'
     The output device supports the complete Unicode repertoire.  This
     directive is useful only for devices that produce character
     entities instead of glyphs.

     If 'unicode' is present, no 'charset' section is required in the
     font description files since the Unicode handling built into
     'groff' is used.  However, if there are entries in a font
     description file's 'charset' section, they either override the
     default mappings for those particular characters or add new
     mappings (normally for composite characters).

     The 'utf8', 'html', and 'xhtml' output devices use this directive.

'unitwidth N'
     Quantities in the font description files are in basic units for
     fonts whose type size is N scaled points.

'unscaled_charwidths'
     Make the font handling module always return unscaled character
     widths.  The 'grohtml' driver uses this directive.

'use_charnames_in_special'
     GNU 'troff' should encode special characters inside device control
     commands; see *note Postprocessor Access::.  The 'grohtml' driver
     uses this directive.

'vert N'
     The vertical motion quantum is N basic units.  All vertical
     quantities are rounded to multiples of N.

'charset'
     This line and everything following it in the file are ignored.  It
     is recognized for compatibility with other 'troff' implementations.
     In GNU 'troff', character set repertoire is described on a per-font
     basis.

   GNU 'troff' recognizes but ignores the directives 'spare1', 'spare2',
and 'biggestfont'.

   The 'res', 'unitwidth', 'fonts', and 'sizes' lines are mandatory.
Directives not listed above are ignored by GNU 'troff' but may be used
by postprocessors to obtain further information about the device.

6.2.2 Font Description File Format
----------------------------------

On typesetting output devices, each font is typically available at
multiple sizes.  While paper measurements in the device description file
are in absolute units, measurements applicable to fonts must be
proportional to the type size.  'groff' achieves this using the
precedent set by AT&T device-independent 'troff': one font size is
chosen as a norm, and all others are scaled linearly relative to that
basis.  The "unit width" is the number of basic units per point when the
font is rendered at this nominal size.

   For instance, 'groff''s 'lbp' device uses a 'unitwidth' of 800.  Its
Times roman font 'TR' has a 'spacewidth' of 833; this is also the width
of its comma, period, centered period, and mathematical asterisk, while
its 'M' is 2,963 basic units.  Thus, an 'M' on the 'lbp' device is 2,963
basic units wide at a notional type size of 800 points.(1)  (*note Font
Description File Format-Footnote-1::)

   A font description file has two sections.  The first is a sequence of
directives, and is parsed similarly to the 'DESC' file described above.
Except for the directive names that begin the second section, their
ordering is immaterial.  Later directives of the same name override
earlier ones, spaces and tabs are handled in the same way, and the same
comment syntax is supported.  Empty lines are ignored throughout.

'name F'
     The name of the font is F.  'DESC' is an invalid font name.  Simple
     integers are valid, but their use is discouraged.(2)  (*note Font
     Description File Format-Footnote-2::)

'spacewidth N'
     The width of an unadjusted inter-word space is N basic units.

   The directives above must appear in the first section; those below
are optional.

'slant N'
     The font's glyphs have a slant of N degrees; a positive N slants in
     the direction of text flow.

'ligatures LIG1 ... LIGN [0]'
     Glyphs LIG1, ..., LIGN are ligatures; possible ligatures are 'ff',
     'fi', 'fl', 'ffi' and 'ffl'.  For compatibility with other 'troff'
     implementations, the list of ligatures may be terminated with
     a '0'.  The list of ligatures must not extend over more than one
     line.

'special'
     The font is "special": when a glyph is requested that is not
     present in the current font, it is sought in any mounted fonts that
     bear this property.

   Other directives in this section are ignored by GNU 'troff', but may
be used by postprocessors to obtain further information about the font.

   The second section contains one or two subsections.  These can appear
in either order; the first one encountered commences the second section.
Each starts with a directive on a line by itself.  A 'charset'
subsection is mandatory unless the associated 'DESC' file contains the
'unicode' directive.  Another subsection, 'kernpairs', is optional.

   The directive 'charset' starts the character set subsection.(3)
(*note Font Description File Format-Footnote-3::) It precedes a series
of glyph descriptions, one per line.  Each such glyph description
comprises a set of fields separated by spaces or tabs and organized as
follows.

     NAME METRICS TYPE CODE [ENTITY-NAME] ['--' COMMENT]

NAME identifies the glyph: if NAME is a printable character C, it
corresponds to the 'troff' ordinary character C.  If NAME is a
multi-character sequence not beginning with '\', it corresponds to the
GNU 'troff' special character escape sequence '\[NAME]'.  A name
consisting of three minus signs, '---', is special and indicates that
the glyph is unnamed: such glyphs can be accessed only by the '\N'
escape sequence in 'troff'.  A special character named '---' can still
be defined using 'char' and similar requests.  The NAME '\-' defines the
minus sign glyph.  Finally, NAME can be the unbreakable one-sixth and
one-twelfth space escape sequences, '\|' and '\^' ("thin" and "hair"
spaces, respectively), in which case only the width metric described
below is interpreted; a font can thus customize the widths of these
spaces.

   The form of the METRICS field is as follows.

     WIDTH[','[HEIGHT[','[DEPTH[','[ITALIC-CORRECTION
       [','[LEFT-ITALIC-CORRECTION[','[SUBSCRIPT-CORRECTION]]]]]]]]]]

There must not be any spaces, tabs, or newlines between these
"subfields" (which have been split here into two lines only for better
legibility).  The subfields are in basic units expressed as decimal
integers.  Unspecified subfields default to '0'.  Since there is no
associated binary format, these values are not required to fit into the
C language data type 'char' as they are in AT&T device-independent
'troff'.

   The WIDTH subfield gives the width of the glyph.  The HEIGHT subfield
gives the height of the glyph (upward is positive); if a glyph does not
extend above the baseline, it should be given a zero height, rather than
a negative height.  The DEPTH subfield gives the depth of the glyph,
that is, the distance below the baseline to which the glyph extends
(downward is positive); if a glyph does not extend below the baseline,
it should be given a zero depth, rather than a negative depth.  Italic
corrections are relevant to glyphs in italic or oblique styles.  The
ITALIC-CORRECTION is the amount of space that should be added after an
oblique glyph to be followed immediately by an upright glyph.  The
LEFT-ITALIC-CORRECTION is the amount of space that should be added
before an oblique glyph to be preceded immediately by an upright glyph.
The SUBSCRIPT-CORRECTION is the amount of space that should be added
after an oblique glyph to be followed by a subscript; it should be less
than the italic correction.

   For fonts used with typesetting devices, the TYPE field gives a
featural description of the glyph: it is a bit mask recording whether
the glyph is an ascender, descender, both, or neither.  When a '\w'
escape sequence is interpolated, these values are bitwise or-ed together
for each glyph and stored in the 'nr' register.  In font descriptions
for terminal devices, all glyphs might have a type of zero, regardless
of their appearance.

'0'
     means the glyph lies entirely between the baseline and a horizontal
     line at the "x-height" of the font; typical examples are 'a', 'c',
     and 'x';

'1'
     means the glyph descends below the baseline, like 'p';

'2'
     means the glyph ascends above the font's x-height, like 'A' or 'b';
     and

'3'
     means the glyph is both an ascender and a descender--this is true
     of parentheses in some fonts.

   The CODE field gives a numeric identifier that the postprocessor uses
to render the glyph.  The glyph can be specified to 'troff' using this
code by means of the '\N' escape sequence.  CODE can be any integer.(4)
(*note Font Description File Format-Footnote-4::)

   The ENTITY-NAME field defines an identifier for the glyph that the
postprocessor uses to print the GNU 'troff' glyph NAME.  This field is
optional; it was introduced so that the 'grohtml' output driver could
encode its character set.  For example, the glyph '\[Po]' is represented
by '&pound;' in HTML 4.0.  For efficiency, these data are now compiled
directly into 'grohtml'.  'grops' uses the field to build sub-encoding
arrays for PostScript fonts containing more than 256 glyphs.  Anything
on the line after the ENTITY-NAME field or '--' is ignored.

   A line in the 'charset' section can also have the form

     NAME "

identifying NAME as another name for the glyph mentioned in the
preceding line.  Such aliases can be chained.

   The directive 'kernpairs' starts a list of kerning adjustments to be
made to adjacent glyph pairs from this font.  It contains a sequence of
lines formatted as follows.

     G1 G2 N

The foregoing means that when glyph G1 is typeset immediately before G2,
the space between them should be increased by N.  Most kerning pairs
should have a negative value for N.

   (1) 800-point type is not practical for most purposes, but using it
enables the quantities in the font description files to be expressed as
integers.

   (2) 'groff' requests and escape sequences interpret non-negative font
names as mounting positions instead.  Further, a font named '0' cannot
be automatically mounted by the 'fonts' directive of a 'DESC' file.

   (3) For typesetter devices, this directive is misnamed since it
starts a list of glyphs, not characters.

   (4) that is, any integer parsable by the C standard library's
'strotol' function

Appendix A Copying This Manual
******************************

                     Version 1.3, 3 November 2008

     Copyright  2000-2018 Free Software Foundation, Inc.
     <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.  We
     recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it can
     be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You accept
     the license if you copy, modify or distribute the work in a way
     requiring permission under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
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     The "Invariant Sections" are certain Secondary Sections whose
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     The "Cover Texts" are certain short passages of text that are
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     A "Transparent" copy of the Document means a machine-readable copy,
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     Opaque formats include proprietary formats that can be read and
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     The "Title Page" means, for a printed book, the title page itself,
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     The "publisher" means any person or entity that distributes copies
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     A section "Entitled XYZ" means a named subunit of the Document
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     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
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     distribute a large enough number of copies you must also follow the
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     You may also lend copies, under the same conditions stated above,
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  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
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     the Document's license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
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     If the required texts for either cover are too voluminous to fit
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     If you publish or distribute Opaque copies of the Document
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     It is requested, but not required, that you contact the authors of
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  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with the
     Modified Version filling the role of the Document, thus licensing
     distribution and modification of the Modified Version to whoever
     possesses a copy of it.  In addition, you must do these things in
     the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of previous
          versions (which should, if there were any, be listed in the
          History section of the Document).  You may use the same title
          as a previous version if the original publisher of that
          version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on the
          Title Page.  If there is no section Entitled "History" in the
          Document, create one stating the title, year, authors, and
          publisher of the Document as given on its Title Page, then add
          an item describing the Modified Version as stated in the
          previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in the
          "History" section.  You may omit a network location for a work
          that was published at least four years before the Document
          itself, or if the original publisher of the version it refers
          to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the section
          all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document, unaltered
          in their text and in their titles.  Section numbers or the
          equivalent are not considered part of the section titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option designate
     some or all of these sections as invariant.  To do this, add their
     titles to the list of Invariant Sections in the Modified Version's
     license notice.  These titles must be distinct from any other
     section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end of
     the list of Cover Texts in the Modified Version.  Only one passage
     of Front-Cover Text and one of Back-Cover Text may be added by (or
     through arrangements made by) any one entity.  If the Document
     already includes a cover text for the same cover, previously added
     by you or by arrangement made by the same entity you are acting on
     behalf of, you may not add another; but you may replace the old
     one, on explicit permission from the previous publisher that added
     the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination all
     of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the documents
     in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that
     document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of a
     storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void,
     and will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly and
     finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, receipt of a copy of some or all of the
     same material does not give you any rights to use it.

  10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     <http://www.gnu.org/copyleft/>.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If the
     Document does not specify a version number of this License, you may
     choose any version ever published (not as a draft) by the Free
     Software Foundation.  If the Document specifies that a proxy can
     decide which future versions of this License can be used, that
     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.

  11. RELICENSING

     "Massive Multiauthor Collaboration Site" (or "MMC Site") means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.
     A "Massive Multiauthor Collaboration" (or "MMC") contained in the
     site means any set of copyrightable works thus published on the MMC
     site.

     "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     "Incorporate" means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is "eligible for relicensing" if it is licensed under this
     License, and if all works that were first published under this
     License somewhere other than this MMC, and subsequently
     incorporated in whole or in part into the MMC, (1) had no cover
     texts or invariant sections, and (2) were thus incorporated prior
     to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.

Appendix B Request Index
************************

Request names appear without a leading control character; the defaults
are '.' for the regular control character and ''' for the no-break
control character.

* Menu:

* ab:                                    Debugging.         (line 12084)
* ad:                                    Manipulating Filling and Adjustment.
                                                            (line  5653)
* af:                                    Assigning Register Formats.
                                                            (line  5378)
* aln:                                   Setting Registers. (line  5282)
* als:                                   Strings.           (line  8924)
* am:                                    Writing Macros.    (line  9484)
* am1:                                   Writing Macros.    (line  9485)
* ami:                                   Writing Macros.    (line  9486)
* ami1:                                  Writing Macros.    (line  9487)
* as:                                    Strings.           (line  8840)
* as1:                                   Strings.           (line  8841)
* asciify:                               Diversions.        (line 11067)
* backtrace:                             Debugging.         (line 12135)
* bd:                                    Artificial Fonts.  (line  8170)
* blm:                                   Blank Line Traps.  (line 10723)
* box:                                   Diversions.        (line 10920)
* boxa:                                  Diversions.        (line 10921)
* bp:                                    Page Control.      (line  7151)
* br:                                    Manipulating Filling and Adjustment.
                                                            (line  5609)
* break:                                 while.             (line  9347)
* brp:                                   Manipulating Filling and Adjustment.
                                                            (line  5726)
* c2:                                    Control Characters.
                                                            (line  4712)
* cc:                                    Control Characters.
                                                            (line  4706)
* ce:                                    Manipulating Filling and Adjustment.
                                                            (line  5776)
* cf:                                    I/O.               (line 11468)
* cflags:                                Using Symbols.     (line  7808)
* ch:                                    Page Location Traps.
                                                            (line 10496)
* char:                                  Using Symbols.     (line  7907)
* chop:                                  Strings.           (line  8871)
* class:                                 Character Classes. (line  7993)
* close:                                 I/O.               (line 11651)
* color:                                 Colors.            (line  8637)
* composite:                             Using Symbols.     (line  7760)
* continue:                              while.             (line  9351)
* cp:                                    Compatibility Mode.
                                                            (line 12376)
* cs:                                    Artificial Fonts.  (line  8200)
* cu:                                    Artificial Fonts.  (line  8161)
* da:                                    Diversions.        (line 10888)
* de:                                    Writing Macros.    (line  9372)
* de1:                                   Writing Macros.    (line  9444)
* defcolor:                              Colors.            (line  8649)
* dei:                                   Writing Macros.    (line  9466)
* dei1:                                  Writing Macros.    (line  9467)
* device:                                Postprocessor Access.
                                                            (line 11686)
* devicem:                               Postprocessor Access.
                                                            (line 11713)
* di:                                    Diversions.        (line 10887)
* do:                                    Compatibility Mode.
                                                            (line 12387)
* ds:                                    ms Document Control Settings.
                                                            (line  1773)
* ds <1>:                                Strings.           (line  8750)
* ds1:                                   Strings.           (line  8751)
* dt:                                    Diversion Traps.   (line 10611)
* ec:                                    Using Escape Sequences.
                                                            (line  4978)
* ecr:                                   Using Escape Sequences.
                                                            (line  5006)
* ecs:                                   Using Escape Sequences.
                                                            (line  5005)
* el:                                    if-else.           (line  9165)
* em:                                    End-of-input Traps.
                                                            (line 10751)
* eo:                                    Using Escape Sequences.
                                                            (line  4973)
* ev:                                    Environments.      (line 11255)
* evc:                                   Environments.      (line 11309)
* ex:                                    Debugging.         (line 12089)
* fam:                                   Font Families.     (line  7379)
* fc:                                    Fields.            (line  6652)
* fchar:                                 Using Symbols.     (line  7908)
* fcolor:                                Colors.            (line  8707)
* fi:                                    Manipulating Filling and Adjustment.
                                                            (line  5636)
* fl:                                    Debugging.         (line 12126)
* fp:                                    Font Positions.    (line  7487)
* fschar:                                Using Symbols.     (line  7909)
* fspecial:                              Special Fonts.     (line  8063)
* ft:                                    Changing Fonts.    (line  7275)
* ft <1>:                                Font Positions.    (line  7542)
* ftr:                                   Changing Fonts.    (line  7323)
* fzoom:                                 Changing Fonts.    (line  7337)
* gcolor:                                Colors.            (line  8679)
* hc:                                    Manipulating Hyphenation.
                                                            (line  5968)
* hcode:                                 Manipulating Hyphenation.
                                                            (line  6175)
* hla:                                   Manipulating Hyphenation.
                                                            (line  6209)
* hlm:                                   Manipulating Hyphenation.
                                                            (line  6222)
* hpf:                                   Manipulating Hyphenation.
                                                            (line  6115)
* hpfa:                                  Manipulating Hyphenation.
                                                            (line  6116)
* hpfcode:                               Manipulating Hyphenation.
                                                            (line  6117)
* hw:                                    Manipulating Hyphenation.
                                                            (line  5901)
* hy:                                    Manipulating Hyphenation.
                                                            (line  6002)
* hym:                                   Manipulating Hyphenation.
                                                            (line  6236)
* hys:                                   Manipulating Hyphenation.
                                                            (line  6251)
* ie:                                    if-else.           (line  9164)
* if:                                    if-then.           (line  9123)
* ig:                                    Comments.          (line  5140)
* in:                                    Line Layout.       (line  6908)
* it:                                    Input Line Traps.  (line 10623)
* itc:                                   Input Line Traps.  (line 10624)
* kern:                                  Ligatures and Kerning.
                                                            (line  8247)
* lc:                                    Leaders.           (line  6606)
* length:                                Strings.           (line  8861)
* lf:                                    Debugging.         (line 12059)
* lg:                                    Ligatures and Kerning.
                                                            (line  8229)
* linetabs:                              Tabs and Fields.   (line  6556)
* ll:                                    Line Layout.       (line  6960)
* ls:                                    Manipulating Spacing.
                                                            (line  6350)
* lsm:                                   Leading Space Traps.
                                                            (line 10733)
* lt:                                    Page Layout.       (line  7114)
* mc:                                    Miscellaneous.     (line 11849)
* mk:                                    Page Motions.      (line  9821)
* mso:                                   I/O.               (line 11459)
* msoquiet:                              I/O.               (line 11460)
* na:                                    Manipulating Filling and Adjustment.
                                                            (line  5719)
* ne:                                    Page Control.      (line  7179)
* nf:                                    Manipulating Filling and Adjustment.
                                                            (line  5644)
* nh:                                    Manipulating Hyphenation.
                                                            (line  6110)
* nm:                                    Miscellaneous.     (line 11738)
* nn:                                    Miscellaneous.     (line 11808)
* nop:                                   if-then.           (line  9142)
* nr:                                    ms Document Control Settings.
                                                            (line  1769)
* nr <1>:                                Setting Registers. (line  5183)
* nr <2>:                                Setting Registers. (line  5237)
* nr <3>:                                Auto-increment.    (line  5331)
* nroff:                                 troff and nroff Modes.
                                                            (line  6811)
* ns:                                    Manipulating Spacing.
                                                            (line  6405)
* nx:                                    I/O.               (line 11501)
* open:                                  I/O.               (line 11618)
* opena:                                 I/O.               (line 11619)
* os:                                    Page Control.      (line  7212)
* output:                                Diversions.        (line 11054)
* pc:                                    Page Layout.       (line  7141)
* pev:                                   Debugging.         (line 12107)
* pi:                                    I/O.               (line 11560)
* pl:                                    Page Layout.       (line  7065)
* pm:                                    Debugging.         (line 12111)
* pn:                                    Page Layout.       (line  7129)
* pnr:                                   Debugging.         (line 12117)
* po:                                    Line Layout.       (line  6882)
* ps:                                    Changing the Type Size.
                                                            (line  8418)
* psbb:                                  Miscellaneous.     (line 11908)
* pso:                                   I/O.               (line 11448)
* ptr:                                   Debugging.         (line 12121)
* pvs:                                   Changing the Vertical Spacing.
                                                            (line  8531)
* rchar:                                 Using Symbols.     (line  7966)
* rd:                                    I/O.               (line 11506)
* return:                                Writing Macros.    (line  9521)
* rfschar:                               Using Symbols.     (line  7967)
* rj:                                    Manipulating Filling and Adjustment.
                                                            (line  5821)
* rm:                                    Strings.           (line  8919)
* rn:                                    Strings.           (line  8916)
* rnn:                                   Setting Registers. (line  5278)
* rr:                                    Setting Registers. (line  5272)
* rs:                                    Manipulating Spacing.
                                                            (line  6406)
* rt:                                    Page Motions.      (line  9822)
* schar:                                 Using Symbols.     (line  7910)
* shc:                                   Manipulating Hyphenation.
                                                            (line  5977)
* shift:                                 Parameters.        (line  9564)
* sizes:                                 Changing the Type Size.
                                                            (line  8476)
* so:                                    I/O.               (line 11419)
* soquiet:                               I/O.               (line 11420)
* sp:                                    Manipulating Spacing.
                                                            (line  6295)
* special:                               Special Fonts.     (line  8062)
* spreadwarn:                            Debugging.         (line 12166)
* ss:                                    Manipulating Filling and Adjustment.
                                                            (line  5830)
* stringdown:                            Strings.           (line  8896)
* stringup:                              Strings.           (line  8897)
* sty:                                   Font Families.     (line  7419)
* substring:                             Strings.           (line  8879)
* sv:                                    Page Control.      (line  7211)
* sy:                                    I/O.               (line 11582)
* ta:                                    Tabs and Fields.   (line  6430)
* tag:                                   Postprocessor Access.
                                                            (line 11726)
* taga:                                  Postprocessor Access.
                                                            (line 11727)
* tc:                                    Tabs and Fields.   (line  6544)
* ti:                                    Line Layout.       (line  6932)
* tkf:                                   Ligatures and Kerning.
                                                            (line  8266)
* tl:                                    Page Layout.       (line  7089)
* tm:                                    Debugging.         (line 12071)
* tm1:                                   Debugging.         (line 12072)
* tmc:                                   Debugging.         (line 12073)
* tr:                                    Character Translations.
                                                            (line  6677)
* trf:                                   I/O.               (line 11467)
* trin:                                  Character Translations.
                                                            (line  6678)
* trnt:                                  Character Translations.
                                                            (line  6766)
* troff:                                 troff and nroff Modes.
                                                            (line  6803)
* uf:                                    Artificial Fonts.  (line  8165)
* ul:                                    Artificial Fonts.  (line  8139)
* unformat:                              Diversions.        (line 11092)
* vpt:                                   Vertical Position Traps.
                                                            (line 10396)
* vs:                                    Changing the Vertical Spacing.
                                                            (line  8490)
* warn:                                  Debugging.         (line 12185)
* warnscale:                             Debugging.         (line 12162)
* wh:                                    Page Location Traps.
                                                            (line 10410)
* while:                                 while.             (line  9285)
* write:                                 I/O.               (line 11630)
* writec:                                I/O.               (line 11631)
* writem:                                I/O.               (line 11642)

Appendix C Escape Sequence Index
********************************

The escape character, '\' by default, is always followed by at least one
more input character, making an escape _sequence_.  Any input token '\X'
with X not in the list below emits a warning and interpolates glyph X.
Note the entries for '\.', which may be obscured by the leader dots, and
for '\newline' and '\space', which are sorted alphabetically, not by
code point order.

* Menu:

* \:                                     Using Escape Sequences.
                                                            (line  4904)
* \ <1>:                                 Using Symbols.     (line  7692)
* \!:                                    Diversions.        (line 11012)
* \":                                    Comments.          (line  5096)
* \#:                                    Comments.          (line  5130)
* \$:                                    Parameters.        (line  9556)
* \$*:                                   Parameters.        (line  9576)
* \$0:                                   Parameters.        (line  9611)
* \$@:                                   Parameters.        (line  9577)
* \$^:                                   Parameters.        (line  9578)
* \%:                                    Manipulating Hyphenation.
                                                            (line  5936)
* \&:                                    Dummy Characters.  (line  8315)
* \':                                    Using Symbols.     (line  7790)
* \(:                                    Using Symbols.     (line  7694)
* \):                                    Dummy Characters.  (line  8363)
* \*:                                    Strings.           (line  8752)
* \,:                                    Ligatures and Kerning.
                                                            (line  8296)
* \-:                                    Using Symbols.     (line  7799)
* \.:                                    Copy Mode.         (line  9693)
* \/:                                    Ligatures and Kerning.
                                                            (line  8286)
* \0:                                    Page Motions.      (line  9968)
* \::                                    Manipulating Hyphenation.
                                                            (line  5937)
* \?:                                    Diversions.        (line 11013)
* \A:                                    Identifiers.       (line  4592)
* \a:                                    Leaders.           (line  6603)
* \B:                                    Numeric Expressions.
                                                            (line  4490)
* \b:                                    Drawing Requests.  (line 10304)
* \c:                                    Line Continuation. (line  7021)
* \C:                                    Using Symbols.     (line  7754)
* \d:                                    Page Motions.      (line  9919)
* \D:                                    Drawing Requests.  (line 10140)
* \e:                                    Using Escape Sequences.
                                                            (line  4959)
* \E:                                    Copy Mode.         (line  9748)
* \f:                                    Changing Fonts.    (line  7276)
* \F:                                    Font Families.     (line  7381)
* \f <1>:                                Font Positions.    (line  7543)
* \g:                                    Assigning Register Formats.
                                                            (line  5436)
* \H:                                    Artificial Fonts.  (line  8088)
* \h:                                    Page Motions.      (line  9936)
* \k:                                    Page Motions.      (line 10036)
* \l:                                    Drawing Requests.  (line 10091)
* \L:                                    Drawing Requests.  (line 10122)
* \m:                                    Colors.            (line  8680)
* \M:                                    Colors.            (line  8708)
* \n:                                    Interpolating Registers.
                                                            (line  5297)
* \n <1>:                                Auto-increment.    (line  5340)
* \N:                                    Using Symbols.     (line  7770)
* \newline:                              Line Continuation. (line  6993)
* \o:                                    Page Motions.      (line 10051)
* \O:                                    Suppressing Output.
                                                            (line 11357)
* \p:                                    Manipulating Filling and Adjustment.
                                                            (line  5727)
* \R:                                    Setting Registers. (line  5184)
* \R <1>:                                Setting Registers. (line  5239)
* \r:                                    Page Motions.      (line  9917)
* \<RET>:                                Line Continuation. (line  6993)
* \S:                                    Artificial Fonts.  (line  8119)
* \s:                                    Changing the Type Size.
                                                            (line  8421)
* \<SP>:                                 Page Motions.      (line  9952)
* \space:                                Page Motions.      (line  9952)
* \t:                                    Tabs and Fields.   (line  6427)
* \u:                                    Page Motions.      (line  9918)
* \v:                                    Page Motions.      (line  9892)
* \V:                                    I/O.               (line 11667)
* \w:                                    Page Motions.      (line  9974)
* \x:                                    Manipulating Spacing.
                                                            (line  6367)
* \X:                                    Postprocessor Access.
                                                            (line 11687)
* \Y:                                    Postprocessor Access.
                                                            (line 11714)
* \z:                                    Page Motions.      (line 10055)
* \Z:                                    Page Motions.      (line 10059)
* \[:                                    Using Symbols.     (line  7694)
* \\:                                    Copy Mode.         (line  9668)
* \^:                                    Page Motions.      (line  9963)
* \_:                                    Using Symbols.     (line  7803)
* \`:                                    Using Symbols.     (line  7795)
* \{:                                    Conditional Blocks.
                                                            (line  9196)
* \{ <1>:                                Conditional Blocks.
                                                            (line  9197)
* \|:                                    Page Motions.      (line  9958)
* \}:                                    Conditional Blocks.
                                                            (line  9197)
* \~:                                    Manipulating Filling and Adjustment.
                                                            (line  5623)

Appendix D Operator Index
*************************

* Menu:

* !:                                     Numeric Expressions.
                                                             (line 4394)
* %:                                     Numeric Expressions.
                                                             (line 4327)
* &:                                     Numeric Expressions.
                                                             (line 4390)
* (:                                     Numeric Expressions.
                                                             (line 4414)
* ):                                     Numeric Expressions.
                                                             (line 4414)
* *:                                     Numeric Expressions.
                                                             (line 4327)
* +:                                     Numeric Expressions.
                                                             (line 4327)
* + <1>:                                 Numeric Expressions.
                                                             (line 4333)
* + (unary):                             Numeric Expressions.
                                                             (line 4425)
* -:                                     Numeric Expressions.
                                                             (line 4327)
* - <1>:                                 Numeric Expressions.
                                                             (line 4333)
* - (unary):                             Numeric Expressions.
                                                             (line 4425)
* /:                                     Numeric Expressions.
                                                             (line 4327)
* ;:                                     Numeric Expressions.
                                                             (line 4358)
* <:                                     Numeric Expressions.
                                                             (line 4384)
* <=:                                    Numeric Expressions.
                                                             (line 4384)
* <?:                                    Numeric Expressions.
                                                             (line 4375)
* <colon>:                               Numeric Expressions.
                                                             (line 4390)
* =:                                     Numeric Expressions.
                                                             (line 4384)
* ==:                                    Numeric Expressions.
                                                             (line 4384)
* >:                                     Numeric Expressions.
                                                             (line 4384)
* >=:                                    Numeric Expressions.
                                                             (line 4384)
* >?:                                    Numeric Expressions.
                                                             (line 4375)
* |:                                     Numeric Expressions.
                                                             (line 4439)

Appendix E Register Index
*************************

The macro package or program a specific register belongs to is appended
in brackets.

   A register name 'x' consisting of exactly one character can be
accessed as '\nx'.  A register name 'xx' consisting of exactly two
characters can be accessed as '\n(xx'.  Register names 'xxx' of any
length can be accessed as '\n[xxx]'.

* Menu:

* $$:                                    Built-in Registers.
                                                            (line  5552)
* %:                                     Page Layout.       (line  7141)
* % <1>:                                 Page Control.      (line  7154)
* .$:                                    Parameters.        (line  9548)
* .A:                                    Built-in Registers.
                                                            (line  5560)
* .a:                                    Manipulating Spacing.
                                                            (line  6368)
* .b:                                    Artificial Fonts.  (line  8172)
* .br:                                   Control Characters.
                                                            (line  4721)
* .c:                                    Built-in Registers.
                                                            (line  5535)
* .C:                                    Compatibility Mode.
                                                            (line 12377)
* .cdp:                                  Environments.      (line 11334)
* .ce:                                   Manipulating Filling and Adjustment.
                                                            (line  5777)
* .cht:                                  Environments.      (line 11333)
* .color:                                Colors.            (line  8638)
* .cp:                                   Compatibility Mode.
                                                            (line 12388)
* .csk:                                  Environments.      (line 11335)
* .d:                                    Diversions.        (line 10947)
* .ev:                                   Environments.      (line 11256)
* .F:                                    Built-in Registers.
                                                            (line  5485)
* .f:                                    Font Positions.    (line  7488)
* .fam:                                  Font Families.     (line  7380)
* .fn:                                   Font Families.     (line  7384)
* .fp:                                   Font Positions.    (line  7489)
* .g:                                    Built-in Registers.
                                                            (line  5556)
* .H:                                    Motion Quanta.     (line  4268)
* .h:                                    Diversions.        (line 10954)
* .height:                               Artificial Fonts.  (line  8091)
* .hla:                                  Manipulating Hyphenation.
                                                            (line  6210)
* .hlc:                                  Manipulating Hyphenation.
                                                            (line  6224)
* .hlm:                                  Manipulating Hyphenation.
                                                            (line  6223)
* .hy:                                   Manipulating Hyphenation.
                                                            (line  6003)
* .hym:                                  Manipulating Hyphenation.
                                                            (line  6237)
* .hys:                                  Manipulating Hyphenation.
                                                            (line  6252)
* .i:                                    Line Layout.       (line  6911)
* .in:                                   Line Layout.       (line  6935)
* .int:                                  Line Continuation. (line  7022)
* .j:                                    Manipulating Filling and Adjustment.
                                                            (line  5654)
* .k:                                    Page Motions.      (line 10047)
* .kern:                                 Ligatures and Kerning.
                                                            (line  8248)
* .L:                                    Manipulating Spacing.
                                                            (line  6351)
* .l:                                    Line Layout.       (line  6963)
* .lg:                                   Ligatures and Kerning.
                                                            (line  8230)
* .linetabs:                             Tabs and Fields.   (line  6557)
* .ll:                                   Line Layout.       (line  6964)
* .lt:                                   Page Layout.       (line  7117)
* .m:                                    Colors.            (line  8683)
* .M:                                    Colors.            (line  8711)
* .n:                                    Environments.      (line 11350)
* .ne:                                   Page Location Traps.
                                                            (line 10547)
* .nm:                                   Miscellaneous.     (line 11740)
* .nn:                                   Miscellaneous.     (line 11809)
* .ns:                                   Manipulating Spacing.
                                                            (line  6407)
* .o:                                    Line Layout.       (line  6885)
* .O:                                    Suppressing Output.
                                                            (line 11410)
* .P:                                    Built-in Registers.
                                                            (line  5564)
* .p:                                    Page Layout.       (line  7068)
* .pe:                                   Page Location Traps.
                                                            (line 10568)
* .pn:                                   Page Layout.       (line  7132)
* .ps:                                   Using Fractional Type Sizes.
                                                            (line  8572)
* .psr:                                  Using Fractional Type Sizes.
                                                            (line  8587)
* .pvs:                                  Changing the Vertical Spacing.
                                                            (line  8534)
* .R:                                    Built-in Registers.
                                                            (line  5488)
* .rj:                                   Manipulating Filling and Adjustment.
                                                            (line  5822)
* .s:                                    Changing the Type Size.
                                                            (line  8422)
* .slant:                                Artificial Fonts.  (line  8120)
* .sr:                                   Using Fractional Type Sizes.
                                                            (line  8588)
* .ss:                                   Manipulating Filling and Adjustment.
                                                            (line  5831)
* .sss:                                  Manipulating Filling and Adjustment.
                                                            (line  5832)
* .sty:                                  Changing Fonts.    (line  7279)
* .T:                                    Built-in Registers.
                                                            (line  5568)
* .t:                                    Page Location Traps.
                                                            (line 10488)
* .tabs:                                 Tabs and Fields.   (line  6431)
* .trunc:                                Page Location Traps.
                                                            (line 10556)
* .U:                                    Built-in Registers.
                                                            (line  5492)
* .u:                                    Manipulating Filling and Adjustment.
                                                            (line  5637)
* .V:                                    Motion Quanta.     (line  4269)
* .v:                                    Changing the Vertical Spacing.
                                                            (line  8493)
* .vpt:                                  Vertical Position Traps.
                                                            (line 10397)
* .w:                                    Environments.      (line 11332)
* .warn:                                 Debugging.         (line 12186)
* .x:                                    Built-in Registers.
                                                            (line  5540)
* .y:                                    Built-in Registers.
                                                            (line  5544)
* .Y:                                    Built-in Registers.
                                                            (line  5548)
* .z:                                    Diversions.        (line 10946)
* .zoom:                                 Changing Fonts.    (line  7338)
* c.:                                    Built-in Registers.
                                                            (line  5536)
* ct:                                    Page Motions.      (line  9979)
* DD [ms]:                               ms Document Control Settings.
                                                            (line  2059)
* DI [ms]:                               ms Document Control Settings.
                                                            (line  2068)
* dl:                                    Diversions.        (line 10973)
* dn:                                    Diversions.        (line 10972)
* dw:                                    Built-in Registers.
                                                            (line  5505)
* dy:                                    Built-in Registers.
                                                            (line  5508)
* FF [ms]:                               ms Document Control Settings.
                                                            (line  1999)
* FI [ms]:                               ms Document Control Settings.
                                                            (line  1992)
* FM [ms]:                               ms Document Control Settings.
                                                            (line  1818)
* FPD [ms]:                              ms Document Control Settings.
                                                            (line  2040)
* FPS [ms]:                              ms Document Control Settings.
                                                            (line  2026)
* FVS [ms]:                              ms Document Control Settings.
                                                            (line  2033)
* GROWPS [ms]:                           ms Document Control Settings.
                                                            (line  1957)
* GS [ms]:                               Differences from AT&T ms.
                                                            (line  3326)
* HM [ms]:                               ms Document Control Settings.
                                                            (line  1811)
* HORPHANS [ms]:                         ms Document Control Settings.
                                                            (line  1968)
* hours:                                 Built-in Registers.
                                                            (line  5502)
* hp:                                    Page Motions.      (line 10044)
* HY [ms]:                               ms Document Control Settings.
                                                            (line  1887)
* LL [ms]:                               ms Document Control Settings.
                                                            (line  1792)
* llx:                                   Miscellaneous.     (line 11909)
* lly:                                   Miscellaneous.     (line 11910)
* ln:                                    Miscellaneous.     (line 11739)
* lsn:                                   Leading Space Traps.
                                                            (line 10734)
* lss:                                   Leading Space Traps.
                                                            (line 10735)
* LT [ms]:                               ms Document Control Settings.
                                                            (line  1801)
* MINGW [ms]:                            ms Document Control Settings.
                                                            (line  2081)
* minutes:                               Built-in Registers.
                                                            (line  5499)
* mo:                                    Built-in Registers.
                                                            (line  5511)
* nl:                                    Page Control.      (line  7224)
* opmaxx:                                Suppressing Output.
                                                            (line 11373)
* opmaxy:                                Suppressing Output.
                                                            (line 11373)
* opminx:                                Suppressing Output.
                                                            (line 11373)
* opminy:                                Suppressing Output.
                                                            (line 11373)
* PD [ms]:                               ms Document Control Settings.
                                                            (line  1916)
* PI [ms]:                               ms Document Control Settings.
                                                            (line  1908)
* PO [ms]:                               ms Document Control Settings.
                                                            (line  1783)
* PORPHANS [ms]:                         ms Document Control Settings.
                                                            (line  1931)
* PS [ms]:                               ms Document Control Settings.
                                                            (line  1873)
* PSINCR [ms]:                           ms Document Control Settings.
                                                            (line  1946)
* QI [ms]:                               ms Document Control Settings.
                                                            (line  1923)
* rsb:                                   Page Motions.      (line  9978)
* rst:                                   Page Motions.      (line  9977)
* sb:                                    Page Motions.      (line  9976)
* seconds:                               Built-in Registers.
                                                            (line  5496)
* skw:                                   Page Motions.      (line  9981)
* slimit:                                Debugging.         (line 12156)
* ssc:                                   Page Motions.      (line  9980)
* st:                                    Page Motions.      (line  9975)
* systat:                                I/O.               (line 11583)
* TC-MARGIN [ms]:                        ms Document Control Settings.
                                                            (line  2089)
* urx:                                   Miscellaneous.     (line 11911)
* ury:                                   Miscellaneous.     (line 11912)
* VS [ms]:                               ms Document Control Settings.
                                                            (line  1880)
* year:                                  Built-in Registers.
                                                            (line  5514)
* yr:                                    Built-in Registers.
                                                            (line  5517)

Appendix F Macro Index
**********************

The macro package a specific macro belongs to is appended in brackets.
They appear without the leading control character (normally '.').

* Menu:

* 1C [ms]:                               ms Multiple Columns.
                                                             (line 3092)
* 2C [ms]:                               ms Multiple Columns.
                                                             (line 3095)
* [ [ms]:                                ms Insertions.      (line 2843)
* ] [ms]:                                ms Insertions.      (line 2844)
* AB [ms]:                               ms Document Description Macros.
                                                             (line 2157)
* AE [ms]:                               ms Document Description Macros.
                                                             (line 2164)
* AI [ms]:                               ms Document Description Macros.
                                                             (line 2140)
* AM [ms]:                               ms Legacy Features. (line 3409)
* AU [ms]:                               ms Document Description Macros.
                                                             (line 2134)
* B [ms]:                                Typeface and decoration.
                                                             (line 2453)
* B1 [ms]:                               ms keeps and displays.
                                                             (line 2742)
* B2 [ms]:                               ms keeps and displays.
                                                             (line 2743)
* BD [ms]:                               ms keeps and displays.
                                                             (line 2783)
* BI [ms]:                               Typeface and decoration.
                                                             (line 2466)
* BT [man]:                              Optional man extensions.
                                                             (line 1412)
* BX [ms]:                               Typeface and decoration.
                                                             (line 2474)
* CD [ms]:                               ms keeps and displays.
                                                             (line 2789)
* CT [man]:                              Optional man extensions.
                                                             (line 1427)
* CW [man]:                              Optional man extensions.
                                                             (line 1430)
* CW [ms]:                               Typeface and decoration.
                                                             (line 2470)
* DA [ms]:                               ms Document Description Macros.
                                                             (line 2147)
* De [man]:                              Optional man extensions.
                                                             (line 1437)
* DE [ms]:                               ms keeps and displays.
                                                             (line 2798)
* Ds [man]:                              Optional man extensions.
                                                             (line 1434)
* DS [ms]:                               ms keeps and displays.
                                                             (line 2773)
* DS [ms] <1>:                           ms keeps and displays.
                                                             (line 2777)
* DS [ms] <2>:                           ms keeps and displays.
                                                             (line 2782)
* DS [ms] <3>:                           ms keeps and displays.
                                                             (line 2788)
* DS [ms] <4>:                           ms keeps and displays.
                                                             (line 2793)
* EE [man]:                              Optional man extensions.
                                                             (line 1444)
* EF [ms]:                               ms Headers and Footers.
                                                             (line 3036)
* EH [ms]:                               ms Headers and Footers.
                                                             (line 3034)
* EN [ms]:                               ms Insertions.      (line 2836)
* EQ [ms]:                               ms Insertions.      (line 2835)
* EX [man]:                              Optional man extensions.
                                                             (line 1440)
* FE [ms]:                               ms Footnotes.       (line 2902)
* FS [ms]:                               ms Footnotes.       (line 2901)
* G [man]:                               Optional man extensions.
                                                             (line 1447)
* GL [man]:                              Optional man extensions.
                                                             (line 1452)
* HB [man]:                              Optional man extensions.
                                                             (line 1457)
* I [ms]:                                Typeface and decoration.
                                                             (line 2463)
* ID [ms]:                               ms keeps and displays.
                                                             (line 2778)
* IP [ms]:                               Paragraphs in ms.   (line 2264)
* KE [ms]:                               ms keeps and displays.
                                                             (line 2730)
* KF [ms]:                               ms keeps and displays.
                                                             (line 2729)
* KS [ms]:                               ms keeps and displays.
                                                             (line 2728)
* LD [ms]:                               ms keeps and displays.
                                                             (line 2774)
* LG [ms]:                               Typeface and decoration.
                                                             (line 2484)
* LP [ms]:                               Paragraphs in ms.   (line 2257)
* MC [ms]:                               ms Multiple Columns.
                                                             (line 3098)
* MS [man]:                              Optional man extensions.
                                                             (line 1465)
* ND [ms]:                               ms Document Description Macros.
                                                             (line 2152)
* NE [man]:                              Optional man extensions.
                                                             (line 1477)
* NH [ms]:                               Headings in ms.     (line 2329)
* NL [ms]:                               Typeface and decoration.
                                                             (line 2496)
* NT [man]:                              Optional man extensions.
                                                             (line 1470)
* OF [ms]:                               ms Headers and Footers.
                                                             (line 3035)
* OH [ms]:                               ms Headers and Footers.
                                                             (line 3033)
* P1 [ms]:                               ms Headers and Footers.
                                                             (line 3045)
* PE [ms]:                               ms Insertions.      (line 2828)
* PF [ms]:                               ms Insertions.      (line 2829)
* PN [man]:                              Optional man extensions.
                                                             (line 1480)
* Pn [man]:                              Optional man extensions.
                                                             (line 1484)
* PP [ms]:                               Paragraphs in ms.   (line 2260)
* PS [ms]:                               ms Insertions.      (line 2827)
* PT [man]:                              Optional man extensions.
                                                             (line 1407)
* PX [ms]:                               ms TOC.             (line 3147)
* QE [ms]:                               Paragraphs in ms.   (line 2277)
* QP [ms]:                               Paragraphs in ms.   (line 2272)
* QS [ms]:                               Paragraphs in ms.   (line 2276)
* R [man]:                               Optional man extensions.
                                                             (line 1490)
* R [ms]:                                Typeface and decoration.
                                                             (line 2459)
* RD [ms]:                               ms keeps and displays.
                                                             (line 2794)
* RE [ms]:                               Indented regions in ms.
                                                             (line 2678)
* RN [man]:                              Optional man extensions.
                                                             (line 1493)
* RP [ms]:                               ms Document Description Macros.
                                                             (line 2115)
* RS [ms]:                               Indented regions in ms.
                                                             (line 2674)
* SH [ms]:                               Headings in ms.     (line 2399)
* SM [ms]:                               Typeface and decoration.
                                                             (line 2490)
* TA [ms]:                               Tab Stops in ms.    (line 3070)
* TB [man]:                              Optional man extensions.
                                                             (line 1462)
* TC [ms]:                               ms TOC.             (line 3152)
* TE [ms]:                               ms Insertions.      (line 2819)
* TL [ms]:                               ms Document Description Macros.
                                                             (line 2129)
* TS [ms]:                               ms Insertions.      (line 2818)
* UL [ms]:                               Typeface and decoration.
                                                             (line 2480)
* VE [man]:                              Optional man extensions.
                                                             (line 1500)
* VS [man]:                              Optional man extensions.
                                                             (line 1496)
* XA [ms]:                               ms TOC.             (line 3136)
* XE [ms]:                               ms TOC.             (line 3137)
* XH [ms]:                               ms TOC.             (line 3188)
* XH-REPLACEMENT [ms]:                   ms TOC.             (line 3197)
* XH-UPDATE-TOC [ms]:                    ms TOC.             (line 3202)
* XN [ms]:                               ms TOC.             (line 3187)
* XN-INIT [ms]:                          ms TOC.             (line 3201)
* XN-REPLACEMENT [ms]:                   ms TOC.             (line 3196)
* XP [ms]:                               Paragraphs in ms.   (line 2283)
* XS [ms]:                               ms TOC.             (line 3135)

Appendix G String Index
***********************

The macro package or program a that defines or uses each string is
appended in brackets.  (Only one string, '.T', is defined by the 'troff'
formatter itself.)  *Note Strings::.

* Menu:

* ! [ms]:                                ms Legacy Features. (line 3448)
* ' [ms]:                                ms Legacy Features. (line 3381)
* ' [ms] <1>:                            ms Legacy Features. (line 3412)
* * [ms]:                                ms Footnotes.       (line 2892)
* , [ms]:                                ms Legacy Features. (line 3399)
* , [ms] <1>:                            ms Legacy Features. (line 3427)
* - [ms]:                                Typographical symbols in ms.
                                                             (line 2236)
* . [ms]:                                ms Legacy Features. (line 3439)
* .T:                                    Strings.            (line 8739)
* .T <1>:                                Strings.            (line 8739)
* / [ms]:                                ms Legacy Features. (line 3430)
* 3 [ms]:                                ms Legacy Features. (line 3457)
* 8 [ms]:                                ms Legacy Features. (line 3451)
* : [ms]:                                ms Legacy Features. (line 3387)
* : [ms] <1>:                            ms Legacy Features. (line 3418)
* < [ms]:                                Typeface and decoration.
                                                             (line 2530)
* > [ms]:                                Typeface and decoration.
                                                             (line 2531)
* ? [ms]:                                ms Legacy Features. (line 3445)
* ^ [ms]:                                ms Legacy Features. (line 3390)
* ^ [ms] <1>:                            ms Legacy Features. (line 3421)
* _ [ms]:                                ms Legacy Features. (line 3436)
* ` [ms]:                                ms Legacy Features. (line 3384)
* ` [ms] <1>:                            ms Legacy Features. (line 3415)
* { [ms]:                                Typeface and decoration.
                                                             (line 2526)
* } [ms]:                                Typeface and decoration.
                                                             (line 2527)
* ~ [ms]:                                ms Legacy Features. (line 3393)
* ~ [ms] <1>:                            ms Legacy Features. (line 3424)
* ABSTRACT [ms]:                         ms language and localization.
                                                             (line 2986)
* ae [ms]:                               ms Legacy Features. (line 3472)
* Ae [ms]:                               ms Legacy Features. (line 3475)
* C [ms]:                                ms Legacy Features. (line 3396)
* CF [ms]:                               ms Document Control Settings.
                                                             (line 1856)
* CH [ms]:                               ms Document Control Settings.
                                                             (line 1835)
* d- [ms]:                               ms Legacy Features. (line 3460)
* D- [ms]:                               ms Legacy Features. (line 3463)
* FAM [ms]:                              ms Document Control Settings.
                                                             (line 1896)
* FR [ms]:                               ms Document Control Settings.
                                                             (line 2047)
* LF [ms]:                               ms Document Control Settings.
                                                             (line 1849)
* LH [ms]:                               ms Document Control Settings.
                                                             (line 1828)
* MONTH1 [ms]:                           ms language and localization.
                                                             (line 2995)
* MONTH10 [ms]:                          ms language and localization.
                                                             (line 3004)
* MONTH11 [ms]:                          ms language and localization.
                                                             (line 3005)
* MONTH12 [ms]:                          ms language and localization.
                                                             (line 3006)
* MONTH2 [ms]:                           ms language and localization.
                                                             (line 2996)
* MONTH3 [ms]:                           ms language and localization.
                                                             (line 2997)
* MONTH4 [ms]:                           ms language and localization.
                                                             (line 2998)
* MONTH5 [ms]:                           ms language and localization.
                                                             (line 2999)
* MONTH6 [ms]:                           ms language and localization.
                                                             (line 3000)
* MONTH7 [ms]:                           ms language and localization.
                                                             (line 3001)
* MONTH8 [ms]:                           ms language and localization.
                                                             (line 3002)
* MONTH9 [ms]:                           ms language and localization.
                                                             (line 3003)
* o [ms]:                                ms Legacy Features. (line 3442)
* oe [ms]:                               ms Legacy Features. (line 3478)
* OE [ms]:                               ms Legacy Features. (line 3481)
* Q [ms]:                                Typographical symbols in ms.
                                                             (line 2239)
* q [ms]:                                ms Legacy Features. (line 3454)
* REFERENCES [ms]:                       ms language and localization.
                                                             (line 2981)
* RF [ms]:                               ms Document Control Settings.
                                                             (line 1863)
* RH [ms]:                               ms Document Control Settings.
                                                             (line 1842)
* SN [ms]:                               Headings in ms.     (line 2378)
* SN-DOT [ms]:                           Headings in ms.     (line 2376)
* SN-NO-DOT [ms]:                        Headings in ms.     (line 2377)
* SN-STYLE [ms]:                         ms Document Control Settings.
                                                             (line 1981)
* SN-STYLE [ms] <1>:                     Headings in ms.     (line 2375)
* th [ms]:                               ms Legacy Features. (line 3466)
* Th [ms]:                               ms Legacy Features. (line 3469)
* TOC [ms]:                              ms language and localization.
                                                             (line 2991)
* U [ms]:                                Typographical symbols in ms.
                                                             (line 2240)
* v [ms]:                                ms Legacy Features. (line 3433)

Appendix H File Keyword Index
*****************************

* Menu:

* #:                                     DESC File Format.  (line 13402)
* # <1>:                                 Font Description File Format.
                                                            (line 13562)
* ---:                                   Font Description File Format.
                                                            (line 13609)
* biggestfont:                           DESC File Format.  (line 13532)
* charset:                               DESC File Format.  (line 13527)
* charset <1>:                           Font Description File Format.
                                                            (line 13601)
* family:                                Changing Fonts.    (line  7279)
* family <1>:                            Font Positions.    (line  7545)
* family <2>:                            DESC File Format.  (line 13406)
* fonts:                                 Using Symbols.     (line  7574)
* fonts <1>:                             Special Fonts.     (line  8063)
* fonts <2>:                             DESC File Format.  (line 13409)
* hor:                                   DESC File Format.  (line 13415)
* image_generator:                       DESC File Format.  (line 13419)
* kernpairs:                             Font Description File Format.
                                                            (line 13697)
* ligatures:                             Font Description File Format.
                                                            (line 13581)
* name:                                  Font Description File Format.
                                                            (line 13566)
* paperlength:                           DESC File Format.  (line 13425)
* papersize:                             DESC File Format.  (line 13429)
* paperwidth:                            DESC File Format.  (line 13456)
* pass_filenames:                        DESC File Format.  (line 13460)
* postpro:                               DESC File Format.  (line 13465)
* prepro:                                DESC File Format.  (line 13468)
* print:                                 DESC File Format.  (line 13472)
* res:                                   DESC File Format.  (line 13476)
* sizes:                                 DESC File Format.  (line 13479)
* sizescale:                             DESC File Format.  (line 13484)
* slant:                                 Font Description File Format.
                                                            (line 13577)
* spacewidth:                            Font Description File Format.
                                                            (line 13571)
* spare1:                                DESC File Format.  (line 13532)
* spare2:                                DESC File Format.  (line 13532)
* special:                               Artificial Fonts.  (line  8189)
* special <1>:                           Font Description File Format.
                                                            (line 13588)
* styles:                                Changing Fonts.    (line  7279)
* styles <1>:                            Font Families.     (line  7434)
* styles <2>:                            Font Positions.    (line  7545)
* styles <3>:                            DESC File Format.  (line 13488)
* tcommand:                              DESC File Format.  (line 13492)
* unicode:                               DESC File Format.  (line 13496)
* unitwidth:                             DESC File Format.  (line 13510)
* unscaled_charwidths:                   DESC File Format.  (line 13514)
* use_charnames_in_special:              Postprocessor Access.
                                                            (line 11705)
* use_charnames_in_special <1>:          DESC File Format.  (line 13518)
* vert:                                  DESC File Format.  (line 13523)

Appendix I Program and File Index
*********************************

* Menu:

* an.tmac:                               man.               (line  1387)
* changebar:                             Miscellaneous.     (line 11882)
* composite.tmac:                        Using Symbols.     (line  7760)
* cp1047.tmac:                           Input Encodings.   (line  3953)
* cs.tmac:                               Manipulating Hyphenation.
                                                            (line  6162)
* de.tmac:                               Manipulating Hyphenation.
                                                            (line  6162)
* DESC:                                  Changing Fonts.    (line  7279)
* DESC <1>:                              Font Families.     (line  7434)
* DESC <2>:                              Font Positions.    (line  7545)
* DESC <3>:                              Using Symbols.     (line  7574)
* DESC <4>:                              Using Symbols.     (line  7780)
* DESC <5>:                              Special Fonts.     (line  8063)
* ec.tmac:                               Input Encodings.   (line  3995)
* en.tmac:                               Manipulating Hyphenation.
                                                            (line  6162)
* eqn:                                   ms Insertions.     (line  2812)
* fr.tmac:                               Manipulating Hyphenation.
                                                            (line  6162)
* freeeuro.pfa:                          Input Encodings.   (line  3995)
* gchem:                                 Groff Options.     (line   482)
* geqn:                                  Groff Options.     (line   482)
* ggrn:                                  Groff Options.     (line   482)
* gpic:                                  Groff Options.     (line   482)
* grap:                                  Groff Options.     (line   482)
* grefer:                                Groff Options.     (line   482)
* groff:                                 Groff Options.     (line   482)
* gsoelim:                               Groff Options.     (line   482)
* gtbl:                                  Groff Options.     (line   482)
* gtroff:                                Groff Options.     (line   482)
* it.tmac:                               Manipulating Hyphenation.
                                                            (line  6162)
* ja.tmac:                               Manipulating Hyphenation.
                                                            (line  6162)
* latin1.tmac:                           Input Encodings.   (line  3958)
* latin2.tmac:                           Input Encodings.   (line  3971)
* latin5.tmac:                           Input Encodings.   (line  3977)
* latin9.tmac:                           Input Encodings.   (line  3982)
* makeindex:                             Indices.           (line  1313)
* man.local:                             Optional man extensions.
                                                            (line  1397)
* man.tmac:                              man.               (line  1387)
* man.ultrix:                            Optional man extensions.
                                                            (line  1421)
* nrchbar:                               Miscellaneous.     (line 11882)
* papersize.tmac:                        Paper Format.      (line   976)
* perl:                                  I/O.               (line 11593)
* pic:                                   ms Insertions.     (line  2812)
* post-grohtml:                          Groff Options.     (line   770)
* pre-grohtml:                           Groff Options.     (line   770)
* preconv:                               Groff Options.     (line   482)
* refer:                                 ms Insertions.     (line  2812)
* soelim:                                Debugging.         (line 12059)
* sv.tmac:                               Manipulating Hyphenation.
                                                            (line  6162)
* tbl:                                   ms Insertions.     (line  2812)
* trace.tmac:                            Writing Macros.    (line  9505)
* troffrc:                               Groff Options.     (line   703)
* troffrc <1>:                           Paper Format.      (line   976)
* troffrc <2>:                           Manipulating Hyphenation.
                                                            (line  6162)
* troffrc <3>:                           Manipulating Hyphenation.
                                                            (line  6210)
* troffrc <4>:                           troff and nroff Modes.
                                                            (line  6803)
* troffrc-end:                           Groff Options.     (line   703)
* troffrc-end <1>:                       Manipulating Hyphenation.
                                                            (line  6210)
* troffrc-end <2>:                       troff and nroff Modes.
                                                            (line  6803)
* tty.tmac:                              troff and nroff Modes.
                                                            (line  6811)
* tty.tmac <1>:                          Line Layout.       (line  6885)
* zh.tmac:                               Manipulating Hyphenation.
                                                            (line  6162)

Appendix J Concept Index
************************

* Menu:

* ", as delimiter:                       Delimiters.        (line  5021)
* ", at end of sentence:                 Sentences.         (line  3630)
* ", at end of sentence <1>:             Using Symbols.     (line  7854)
* ", embedding in a macro argument:      Calling Macros.    (line  4835)
* %, as delimiter:                       Delimiters.        (line  5052)
* &, as delimiter:                       Delimiters.        (line  5052)
* ', as a comment:                       Comments.          (line  5124)
* ', as delimiter:                       Delimiters.        (line  5021)
* ', at end of sentence:                 Sentences.         (line  3630)
* ', at end of sentence <1>:             Using Symbols.     (line  7854)
* (, as delimiter:                       Delimiters.        (line  5052)
* ), as delimiter:                       Delimiters.        (line  5052)
* ), at end of sentence:                 Sentences.         (line  3630)
* ), at end of sentence <1>:             Using Symbols.     (line  7854)
* *, as delimiter:                       Delimiters.        (line  5052)
* *, at end of sentence:                 Sentences.         (line  3630)
* *, at end of sentence <1>:             Using Symbols.     (line  7854)
* +, and page motion:                    Numeric Expressions.
                                                            (line  4425)
* +, as delimiter:                       Delimiters.        (line  5052)
* -, and page motion:                    Numeric Expressions.
                                                            (line  4425)
* -, as delimiter:                       Delimiters.        (line  5052)
* ., as delimiter:                       Delimiters.        (line  5050)
* .h register, difference to nl:         Diversions.        (line 10967)
* .ps register, in comparison with .psr: Using Fractional Type Sizes.
                                                            (line  8588)
* .s register, in comparison with .sr:   Using Fractional Type Sizes.
                                                            (line  8588)
* .S register, Plan 9 alias for .tabs:   Tabs and Fields.   (line  6535)
* .t register, and diversions:           Diversion Traps.   (line 10611)
* .tabs register, Plan 9 alias (.S):     Tabs and Fields.   (line  6535)
* .V register, and vs:                   Changing the Vertical Spacing.
                                                            (line  8496)
* /, as delimiter:                       Delimiters.        (line  5052)
* 8-bit input:                           Font Description File Format.
                                                            (line 13609)
* <, as delimiter:                       Delimiters.        (line  5052)
* <colon>, as delimiter:                 Delimiters.        (line  5052)
* =, as delimiter:                       Delimiters.        (line  5052)
* >, as delimiter:                       Delimiters.        (line  5052)
* [, macro names starting with, and refer: Identifiers.     (line  4587)
* \!, and copy mode:                     Diversions.        (line 11021)
* \!, and output request:                Diversions.        (line 11053)
* \!, and trnt:                          Character Translations.
                                                            (line  6766)
* \!, as delimiter:                      Delimiters.        (line  5030)
* \!, as delimiter <1>:                  Delimiters.        (line  5056)
* \!, in top-level diversion:            Diversions.        (line 11045)
* \!, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \!, incompatibilities with AT&T troff <1>: Other Differences.
                                                            (line 12596)
* \$, when reading text for a macro:     Copy Mode.         (line  9653)
* \%, and translations:                  Character Translations.
                                                            (line  6695)
* \%, as delimiter:                      Delimiters.        (line  5030)
* \%, as delimiter <1>:                  Delimiters.        (line  5056)
* \%, following \X or \Y:                Manipulating Hyphenation.
                                                            (line  5946)
* \%, in \X:                             Postprocessor Access.
                                                            (line 11694)
* \%, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \&, and glyph definitions:             Using Symbols.     (line  7910)
* \&, and translations:                  Character Translations.
                                                            (line  6706)
* \&, as delimiter:                      Delimiters.        (line  5030)
* \&, at end of sentence:                Sentences.         (line  3614)
* \&, in \X:                             Postprocessor Access.
                                                            (line 11694)
* \&, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \', and translations:                  Character Translations.
                                                            (line  6689)
* \', as delimiter:                      Delimiters.        (line  5030)
* \', as delimiter <1>:                  Delimiters.        (line  5056)
* \', incompatibilities with AT&T troff: Other Differences. (line 12508)
* \(, and translations:                  Character Translations.
                                                            (line  6689)
* \), as delimiter:                      Delimiters.        (line  5030)
* \), in \X:                             Postprocessor Access.
                                                            (line 11694)
* \*, and warnings:                      Warnings.          (line 12260)
* \*, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12366)
* \*, when reading text for a macro:     Copy Mode.         (line  9653)
* \, disabling (eo):                     Using Escape Sequences.
                                                            (line  4973)
* \, embedding in a macro argument:      Calling Macros.    (line  4835)
* \,, as delimiter:                      Delimiters.        (line  5030)
* \- glyph, and cflags:                  Using Symbols.     (line  7837)
* \-, and translations:                  Character Translations.
                                                            (line  6689)
* \-, as delimiter:                      Delimiters.        (line  5030)
* \-, as delimiter <1>:                  Delimiters.        (line  5056)
* \-, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \/, as delimiter:                      Delimiters.        (line  5030)
* \/, as delimiter <1>:                  Delimiters.        (line  5056)
* \0, as delimiter:                      Delimiters.        (line  5030)
* \:, as delimiter:                      Delimiters.        (line  5030)
* \:, as delimiter <1>:                  Delimiters.        (line  5056)
* \<colon>, in \X:                       Postprocessor Access.
                                                            (line 11694)
* \?, and copy mode:                     Operators in Conditionals.
                                                            (line  9068)
* \?, and copy mode <1>:                 Diversions.        (line 11021)
* \?, as delimiter:                      Delimiters.        (line  5030)
* \?, in top-level diversion:            Diversions.        (line 11050)
* \?, incompatibilities with AT&T troff: Other Differences. (line 12596)
* \a, and copy mode:                     Leaders.           (line  6603)
* \a, and translations:                  Character Translations.
                                                            (line  6699)
* \a, as delimiter:                      Delimiters.        (line  5030)
* \A, delimiters allowed by:             Delimiters.        (line  5036)
* \A, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \b, delimiters allowed by:             Delimiters.        (line  5036)
* \b, limitations of:                    Drawing Requests.  (line 10312)
* \C, and translations:                  Character Translations.
                                                            (line  6689)
* \c, as delimiter:                      Delimiters.        (line  5030)
* \c, as delimiter <1>:                  Delimiters.        (line  5056)
* \c, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \c, when filling disabled:             Line Continuation. (line  7036)
* \c, when filling enabled:              Line Continuation. (line  7028)
* \D'f ...' and horizontal motion quantum: Drawing Requests.
                                                            (line 10223)
* \d, as delimiter:                      Delimiters.        (line  5030)
* \D, delimiters allowed by:             Delimiters.        (line  5047)
* \e, and glyph definitions:             Using Symbols.     (line  7910)
* \e, and translations:                  Character Translations.
                                                            (line  6693)
* \e, as delimiter:                      Delimiters.        (line  5030)
* \E, as delimiter:                      Delimiters.        (line  5030)
* \e, as delimiter <1>:                  Delimiters.        (line  5056)
* \e, incompatibilities with AT&T troff: Other Differences. (line 12596)
* \F, and changing fonts:                Changing Fonts.    (line  7279)
* \F, and font positions:                Font Positions.    (line  7545)
* \f, and font translations:             Changing Fonts.    (line  7323)
* \f, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12468)
* \h, delimiters allowed by:             Delimiters.        (line  5047)
* \H, delimiters allowed by:             Delimiters.        (line  5047)
* \H, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12468)
* \H, using + and - with:                Numeric Expressions.
                                                            (line  4431)
* \H, with fractional type sizes:        Using Fractional Type Sizes.
                                                            (line  8554)
* \l, and glyph definitions:             Using Symbols.     (line  7910)
* \L, and glyph definitions:             Using Symbols.     (line  7910)
* \l, delimiters allowed by:             Delimiters.        (line  5047)
* \L, delimiters allowed by:             Delimiters.        (line  5047)
* \N, and translations:                  Character Translations.
                                                            (line  6689)
* \n, and warnings:                      Warnings.          (line 12272)
* \N, delimiters allowed by:             Delimiters.        (line  5047)
* \n, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12366)
* \n, when reading text for a macro:     Copy Mode.         (line  9653)
* \o, delimiters allowed by:             Delimiters.        (line  5036)
* \p, as delimiter:                      Delimiters.        (line  5030)
* \p, as delimiter <1>:                  Delimiters.        (line  5056)
* \R, after \c:                          Line Continuation. (line  7022)
* \R, and warnings:                      Warnings.          (line 12272)
* \r, as delimiter:                      Delimiters.        (line  5030)
* \R, delimiters allowed by:             Delimiters.        (line  5047)
* \R, difference to nr:                  Auto-increment.    (line  5331)
* \R, using + and - with:                Numeric Expressions.
                                                            (line  4431)
* \<RET>, when reading text for a macro: Copy Mode.         (line  9653)
* \s, delimiters allowed by:             Delimiters.        (line  5047)
* \S, delimiters allowed by:             Delimiters.        (line  5047)
* \s, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12468)
* \S, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12468)
* \s, incompatibilities with AT&T troff <1>: Compatibility Mode.
                                                            (line 12481)
* \s, using + and - with:                Numeric Expressions.
                                                            (line  4431)
* \s, with fractional type sizes:        Using Fractional Type Sizes.
                                                            (line  8554)
* \<SP>, as delimiter:                   Delimiters.        (line  5030)
* \<SP>, difference to \~:               Calling Macros.    (line  4827)
* \<SP>, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12508)
* \t, and copy mode:                     Tabs and Fields.   (line  6427)
* \t, and translations:                  Character Translations.
                                                            (line  6699)
* \t, and warnings:                      Warnings.          (line 12278)
* \t, as delimiter:                      Delimiters.        (line  5030)
* \u, as delimiter:                      Delimiters.        (line  5030)
* \V, and copy mode:                     I/O.               (line 11669)
* \v, delimiters allowed by:             Delimiters.        (line  5047)
* \v, internal representation:           Gtroff Internals.  (line 11975)
* \w, delimiters allowed by:             Delimiters.        (line  5036)
* \X, and special characters:            Postprocessor Access.
                                                            (line 11705)
* \X, delimiters allowed by:             Delimiters.        (line  5036)
* \x, delimiters allowed by:             Delimiters.        (line  5047)
* \X, followed by \%:                    Manipulating Hyphenation.
                                                            (line  5946)
* \Y, followed by \%:                    Manipulating Hyphenation.
                                                            (line  5946)
* \Z, delimiters allowed by:             Delimiters.        (line  5036)
* \[, and translations:                  Character Translations.
                                                            (line  6689)
* \\, when reading text for a macro:     Copy Mode.         (line  9692)
* \^, as delimiter:                      Delimiters.        (line  5030)
* \^, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \_, and translations:                  Character Translations.
                                                            (line  6689)
* \_, as delimiter:                      Delimiters.        (line  5030)
* \_, as delimiter <1>:                  Delimiters.        (line  5056)
* \_, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \`, and translations:                  Character Translations.
                                                            (line  6689)
* \`, as delimiter:                      Delimiters.        (line  5030)
* \`, as delimiter <1>:                  Delimiters.        (line  5056)
* \`, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \{, as delimiter:                      Delimiters.        (line  5030)
* \{, as delimiter <1>:                  Delimiters.        (line  5056)
* \{, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \|, as delimiter:                      Delimiters.        (line  5030)
* \|, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \}, and warnings:                      Warnings.          (line 12283)
* \}, as delimiter:                      Delimiters.        (line  5030)
* \}, as delimiter <1>:                  Delimiters.        (line  5056)
* \}, incompatibilities with AT&T troff: Other Differences. (line 12508)
* \~, and translations:                  Character Translations.
                                                            (line  6695)
* \~, as delimiter:                      Delimiters.        (line  5030)
* \~, difference to \<SP>:               Calling Macros.    (line  4827)
* \~, incompatibilities with AT&T troff: Other Differences. (line 12501)
* ], as part of an identifier:           Identifiers.       (line  4580)
* ], at end of sentence:                 Sentences.         (line  3630)
* ], at end of sentence <1>:             Using Symbols.     (line  7854)
* ], macro names starting with, and refer: Identifiers.     (line  4587)
* |, and page motion:                    Numeric Expressions.
                                                            (line  4439)
* ab request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12551)
* aborting (ab):                         Debugging.         (line 12084)
* absolute (sic) position operator (|):  Numeric Expressions.
                                                            (line  4439)
* accent marks [ms]:                     ms Legacy Features.
                                                            (line  3371)
* access to postprocessor:               Postprocessor Access.
                                                            (line 11677)
* accessing unnamed glyphs with \N:      Font Description File Format.
                                                            (line 13609)
* activating kerning (kern):             Ligatures and Kerning.
                                                            (line  8248)
* activating ligatures (lg):             Ligatures and Kerning.
                                                            (line  8230)
* activating track kerning (tkf):        Ligatures and Kerning.
                                                            (line  8266)
* ad request, and hyphenation margin:    Manipulating Hyphenation.
                                                            (line  6237)
* ad request, and hyphenation space:     Manipulating Hyphenation.
                                                            (line  6252)
* addition:                              Numeric Expressions.
                                                            (line  4327)
* additional inter-sentence space:       Manipulating Filling and Adjustment.
                                                            (line  5843)
* adjustment and filling, manipulating:  Manipulating Filling and Adjustment.
                                                            (line  5577)
* adjustment mode register (.j):         Manipulating Filling and Adjustment.
                                                            (line  5710)
* adjustment to both margins, difference from AT&T troff: Other Differences.
                                                            (line 12514)
* Adobe Glyph List (AGL):                Using Symbols.     (line  7649)
* alias, diversion, creating (als):      Strings.           (line  8924)
* alias, diversion, removing (rm):       Strings.           (line  8959)
* alias, macro, creating (als):          Strings.           (line  8924)
* alias, macro, removing (rm):           Strings.           (line  8959)
* alias, register, creation (aln):       Setting Registers. (line  5282)
* alias, register, removing (aln):       Setting Registers. (line  5288)
* alias, string, creating (als):         Strings.           (line  8924)
* alias, string, removing (rm):          Strings.           (line  8959)
* als request, and \$0:                  Parameters.        (line  9611)
* am, am1, ami requests, and warnings:   Warnings.          (line 12260)
* annotations:                           Footnotes and Annotations.
                                                            (line  1284)
* appending to a diversion (da):         Diversions.        (line 10888)
* appending to a file (opena):           I/O.               (line 11619)
* appending to a macro (am):             Writing Macros.    (line  9487)
* appending to a string (as):            Strings.           (line  8841)
* approximation output register (.A):    Built-in Registers.
                                                            (line  5560)
* arc, drawing (\D'a ...'):              Drawing Requests.  (line 10200)
* argument:                              Requests and Macros.
                                                            (line  3796)
* arguments to macros:                   Calling Macros.    (line  4811)
* arguments to macros, and tabs:         Invoking Requests. (line  4749)
* arguments to requests:                 Invoking Requests. (line  4749)
* arguments to requests, and tabs:       Invoking Requests. (line  4749)
* arguments, and compatibility mode:     Gtroff Internals.  (line 12012)
* arguments, for escape sequences, delimiting: Delimiters.  (line  5021)
* arguments, of strings:                 Strings.           (line  8754)
* arithmetic operators:                  Numeric Expressions.
                                                            (line  4327)
* artificial fonts:                      Artificial Fonts.  (line  8081)
* as, as1 requests, and comments:        Comments.          (line  5099)
* as, as1 requests, and warnings:        Warnings.          (line 12260)
* ASCII, output encoding:                Groff Options.     (line   747)
* asciify request, and writem:           I/O.               (line 11642)
* assertion (arithmetic operator):       Numeric Expressions.
                                                            (line  4333)
* assigning formats (af):                Assigning Register Formats.
                                                            (line  5372)
* assignments, indirect:                 Interpolating Registers.
                                                            (line  5299)
* assignments, nested:                   Interpolating Registers.
                                                            (line  5299)
* AT&T ms, macro package differences:    Differences from AT&T ms.
                                                            (line  3242)
* auto-incrementation of a register:     Auto-increment.    (line  5323)
* automatic hyphenation:                 Manipulating Hyphenation.
                                                            (line  5886)
* automatic hyphenation parameters:      Manipulating Hyphenation.
                                                            (line  5988)
* auxiliary macro package:               Major Macro Packages.
                                                            (line  1378)
* available glyphs, list (groff_char(7) man page): Using Symbols.
                                                            (line  7637)
* background:                            Background.        (line   227)
* background color name register (.M):   Colors.            (line  8720)
* backslash glyph, formatting (\[rs]):   Using Escape Sequences.
                                                            (line  4961)
* backslash, embedding in a macro argument: Calling Macros. (line  4835)
* backslash, printing (\\, \e, \E, \[rs]): Other Differences.
                                                            (line 12596)
* backspace character, and translations: Character Translations.
                                                            (line  6699)
* backtrace of input stack (backtrace):  Debugging.         (line 12135)
* baseline, text:                        Page Geometry.     (line  4152)
* baseline, text <1>:                    Manipulating Type Size and Vertical Spacing.
                                                            (line  8394)
* basic scaling unit (u):                Measurements.      (line  4213)
* basic units:                           Page Geometry.     (line  4137)
* basic units, conversion to:            Measurements.      (line  4207)
* basics of macros:                      Basics.            (line  1059)
* bd request, and font styles:           Font Families.     (line  7419)
* bd request, and font translations:     Changing Fonts.    (line  7323)
* bd request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12568)
* begin of conditional block (\{):       Conditional Blocks.
                                                            (line  9197)
* beginning diversion (di):              Diversions.        (line 10888)
* blank line:                            Breaking.          (line  3716)
* blank line (sp):                       Basics.            (line  1145)
* blank line macro (blm):                Breaking.          (line  3716)
* blank line macro (blm) <1>:            Invoking Requests. (line  4780)
* blank line macro (blm) <2>:            Blank Line Traps.  (line 10723)
* blank line trap (blm):                 Invoking Requests. (line  4780)
* blank line traps:                      Blank Line Traps.  (line 10722)
* blank lines, disabling:                Manipulating Spacing.
                                                            (line  6407)
* block, conditional, begin (\{):        Conditional Blocks.
                                                            (line  9197)
* block, conditional, end (\}):          Conditional Blocks.
                                                            (line  9197)
* blocks, conditional:                   Conditional Blocks.
                                                            (line  9195)
* body, of a while request:              while.             (line  9284)
* boldface, imitating (bd):              Artificial Fonts.  (line  8172)
* bottom margin:                         Page Layout.       (line  7074)
* boundary-relative motion operator (|): Numeric Expressions.
                                                            (line  4439)
* bounding box:                          Miscellaneous.     (line 11912)
* box (diversion type):                  Diversions.        (line 10915)
* box request, and warnings:             Warnings.          (line 12255)
* box rule glyph (\[br]):                Drawing Requests.  (line 10123)
* box, boxa requests, and warnings:      Warnings.          (line 12260)
* boxa request, and dn (dl):             Diversions.        (line 10973)
* boxa request, and warnings:            Warnings.          (line 12255)
* boxes [ms]:                            ms keeps and displays.
                                                            (line  2739)
* bp request, and top-level diversion:   Page Control.      (line  7170)
* bp request, and traps (.pe):           Page Location Traps.
                                                            (line 10568)
* bp request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* bp request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12555)
* bp request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* br glyph, and cflags:                  Using Symbols.     (line  7849)
* brace escape sequence, closing (\}):   Conditional Blocks.
                                                            (line  9197)
* brace escape sequence, opening (\}):   Conditional Blocks.
                                                            (line  9197)
* brace escape sequences (\{, \}):       Conditional Blocks.
                                                            (line  9197)
* break:                                 Basics.            (line  1102)
* break <1>:                             Breaking.          (line  3692)
* break <2>:                             Manipulating Filling and Adjustment.
                                                            (line  5577)
* break (br):                            Basics.            (line  1169)
* break request, in a while loop:        while.             (line  9347)
* break, page:                           Page Geometry.     (line  4176)
* breaking file names (\:):              Manipulating Hyphenation.
                                                            (line  5953)
* breaking URLs (\:):                    Manipulating Hyphenation.
                                                            (line  5953)
* breaking without hyphens (\:):         Manipulating Hyphenation.
                                                            (line  5953)
* built-in registers:                    Built-in Registers.
                                                            (line  5467)
* bulleted list, example markup [ms]:    Lists in ms.       (line  2558)
* c scaling unit:                        Measurements.      (line  4220)
* calling a macro:                       Requests and Macros.
                                                            (line  3807)
* calling macros:                        Calling Macros.    (line  4811)
* capabilities of groff:                 groff Capabilities.
                                                            (line   279)
* case-transforming a string (stringdown, stringup): Strings.
                                                            (line  8897)
* categories, warning:                   Warnings.          (line 12209)
* ce request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* ce request, difference from .ad c:     Manipulating Filling and Adjustment.
                                                            (line  5784)
* centered text (filled):                Manipulating Filling and Adjustment.
                                                            (line  5664)
* centered text (unfilled):              Manipulating Filling and Adjustment.
                                                            (line  5777)
* centering lines (ce):                  Basics.            (line  1157)
* centering lines (ce) <1>:              Manipulating Filling and Adjustment.
                                                            (line  5777)
* centimeter scaling unit (c):           Measurements.      (line  4220)
* cf request, and copy mode:             I/O.               (line 11468)
* cf request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* changing control characters:           Control Characters.
                                                            (line  4690)
* changing font family (fam, \F):        Font Families.     (line  7384)
* changing font position (\f):           Font Positions.    (line  7545)
* changing font style (sty):             Font Families.     (line  7419)
* changing fonts (ft, \f):               Changing Fonts.    (line  7279)
* changing format, and read-only registers: Assigning Register Formats.
                                                            (line  5431)
* changing the font height (\H):         Artificial Fonts.  (line  8091)
* changing the font slant (\S):          Artificial Fonts.  (line  8120)
* changing the page number character (pc): Page Layout.     (line  7141)
* changing trap location (ch):           Page Location Traps.
                                                            (line 10496)
* changing type sizes (ps, \s):          Changing the Type Size.
                                                            (line  8422)
* changing vertical line spacing (vs):   Changing the Vertical Spacing.
                                                            (line  8493)
* char request, and soft hyphen character: Manipulating Hyphenation.
                                                            (line  5977)
* char request, and translations:        Character Translations.
                                                            (line  6689)
* char request, used with \N:            Using Symbols.     (line  7770)
* character:                             Using Symbols.     (line  7566)
* character class (class):               Character Classes. (line  7993)
* character classes:                     Character Classes. (line  7987)
* character properties (cflags):         Using Symbols.     (line  7808)
* character translations:                Character Translations.
                                                            (line  6670)
* character, backspace, and translations: Character Translations.
                                                            (line  6699)
* character, control (.):                Requests and Macros.
                                                            (line  3780)
* character, control, changing (cc):     Control Characters.
                                                            (line  4702)
* character, defining (char):            Using Symbols.     (line  7910)
* character, defining fallback (fchar, fschar, schar): Using Symbols.
                                                            (line  7910)
* character, distinguished from glyph:   Using Symbols.     (line  7566)
* character, dummy (\&):                 Dummy Characters.  (line  8315)
* character, dummy (\&), as control character suppressor: Requests and Macros.
                                                            (line  3787)
* character, dummy (\&), effect on kerning: Ligatures and Kerning.
                                                            (line  8253)
* character, dummy (\&), effect on \l escape: Drawing Requests.
                                                            (line 10107)
* character, escape, changing (ec):      Using Escape Sequences.
                                                            (line  4978)
* character, escape, while defining glyph: Using Symbols.   (line  7910)
* character, field delimiting (fc):      Fields.            (line  6640)
* character, field padding (fc):         Fields.            (line  6640)
* character, horizontal tab:             Tabs and Leaders.  (line  3752)
* character, hyphenation (\%):           Manipulating Hyphenation.
                                                            (line  5937)
* character, leader:                     Tabs and Leaders.  (line  3752)
* character, leader repetition (lc):     Leaders.           (line  6606)
* character, leader, and translations:   Character Translations.
                                                            (line  6699)
* character, leader, non-interpreted (\a): Leaders.         (line  6603)
* character, named (\C):                 Using Symbols.     (line  7754)
* character, newline, and translations:  Character Translations.
                                                            (line  6699)
* character, no-break control ('):       Requests and Macros.
                                                            (line  3780)
* character, no-break control, changing (c2): Control Characters.
                                                            (line  4702)
* character, soft hyphen, setting (shc): Manipulating Hyphenation.
                                                            (line  5977)
* character, special:                    Character Translations.
                                                            (line  6689)
* character, tab repetition (tc):        Tabs and Fields.   (line  6544)
* character, tab, and translations:      Character Translations.
                                                            (line  6699)
* character, tab, non-interpreted (\t):  Tabs and Fields.   (line  6427)
* character, transparent:                Using Symbols.     (line  7854)
* character, transparent dummy (\)):     Dummy Characters.  (line  8363)
* characters, end-of-sentence:           Using Symbols.     (line  7825)
* characters, end-of-sentence transparent: Sentences.       (line  3630)
* characters, hyphenation:               Using Symbols.     (line  7830)
* characters, input, and output glyphs, compatibility with AT&T troff: Other Differences.
                                                            (line 12568)
* characters, invalid for trf request:   I/O.               (line 11490)
* characters, invalid input:             Identifiers.       (line  4545)
* characters, overlapping:               Using Symbols.     (line  7844)
* characters, special:                   Sentences.         (line  3630)
* characters, unnamed, accessing with \N: Font Description File Format.
                                                            (line 13609)
* circle, drawing (\D'c ...'):           Drawing Requests.  (line 10181)
* circle, solid, drawing (\D'C ...'):    Drawing Requests.  (line 10186)
* class of characters (class):           Character Classes. (line  7993)
* classes, character:                    Character Classes. (line  7987)
* clearing input line trap (it, itc):    Input Line Traps.  (line 10624)
* closing brace escape sequence (\}):    Conditional Blocks.
                                                            (line  9197)
* closing file (close):                  I/O.               (line 11651)
* code page 1047, input encoding:        Input Encodings.   (line  3953)
* code page 1047, output encoding:       Groff Options.     (line   759)
* code, hyphenation (hcode):             Manipulating Hyphenation.
                                                            (line  6175)
* color name, background, register (.M): Colors.            (line  8720)
* color name, fill, register (.M):       Colors.            (line  8720)
* color name, stroke, register (.m):     Colors.            (line  8694)
* color, default:                        Colors.            (line  8670)
* color, fill:                           Colors.            (line  8628)
* color, stroke:                         Colors.            (line  8628)
* colors:                                Colors.            (line  8628)
* colors, fill, unnamed (\D'F...'):      Drawing Requests.  (line 10288)
* command prefix:                        Environment.       (line   839)
* command-line options:                  Groff Options.     (line   524)
* comments:                              Comments.          (line  5092)
* comments in device description files:  DESC File Format.  (line 13402)
* comments in font description files:    Font Description File Format.
                                                            (line 13562)
* comments, lining up with tabs:         Comments.          (line  5105)
* comments, with ds:                     Strings.           (line  8775)
* common features:                       Common Features.   (line  1182)
* common name space of macros, diversions, and strings: Identifiers.
                                                            (line  4632)
* comparison of strings:                 Operators in Conditionals.
                                                            (line  9060)
* comparison operators:                  Numeric Expressions.
                                                            (line  4384)
* compatibility mode:                    Warnings.          (line 12300)
* compatibility mode <1>:                Compatibility Mode.
                                                            (line 12366)
* compatibility mode, and parameters:    Gtroff Internals.  (line 12012)
* complementation, logical:              Numeric Expressions.
                                                            (line  4394)
* composite glyph names:                 Using Symbols.     (line  7649)
* conditional block, begin (\{):         Conditional Blocks.
                                                            (line  9197)
* conditional block, end (\}):           Conditional Blocks.
                                                            (line  9197)
* conditional blocks:                    Conditional Blocks.
                                                            (line  9195)
* conditional expressions:               Operators in Conditionals.
                                                            (line  8979)
* conditional output for terminal (TTY): Operators in Conditionals.
                                                            (line  9005)
* conditional page break (ne):           Page Control.      (line  7179)
* conditionals and loops:                Conditionals and Loops.
                                                            (line  8972)
* configuring control characters:        Control Characters.
                                                            (line  4690)
* consecutive hyphenated lines (hlm):    Manipulating Hyphenation.
                                                            (line  6224)
* constant glyph space mode (cs):        Artificial Fonts.  (line  8200)
* contents, table of:                    Table of Contents. (line  1299)
* contents, table of <1>:                Leaders.           (line  6616)
* continuation, input line (\<RET>):     Line Continuation. (line  6993)
* continuation, output line (\c):        Line Continuation. (line  7022)
* continue request, in a while loop:     while.             (line  9347)
* continued output line register (.int): Line Continuation. (line  7047)
* continuous underlining (cu):           Artificial Fonts.  (line  8161)
* control character (.):                 Requests and Macros.
                                                            (line  3780)
* control character, changing (cc):      Control Characters.
                                                            (line  4702)
* control character, no-break ('):       Requests and Macros.
                                                            (line  3780)
* control character, no-break, changing (c2): Control Characters.
                                                            (line  4702)
* control characters:                    Control Characters.
                                                            (line  4690)
* control line:                          Requests and Macros.
                                                            (line  3792)
* control, line:                         Line Continuation. (line  6988)
* control, page:                         Page Control.      (line  7150)
* conventions for input:                 Input Conventions. (line  4019)
* conversion to basic units:             Measurements.      (line  4207)
* copy mode:                             Copy Mode.         (line  9653)
* copy mode <1>:                         Copy Mode.         (line  9653)
* copy mode, and cf request:             I/O.               (line 11468)
* copy mode, and device request:         Postprocessor Access.
                                                            (line 11692)
* copy mode, and length request:         Strings.           (line  8861)
* copy mode, and macro parameters:       Parameters.        (line  9570)
* copy mode, and output request:         Diversions.        (line 11053)
* copy mode, and trf request:            I/O.               (line 11468)
* copy mode, and write request:          I/O.               (line 11631)
* copy mode, and writec request:         I/O.               (line 11631)
* copy mode, and writem request:         I/O.               (line 11645)
* copy mode, and \!:                     Diversions.        (line 11021)
* copy mode, and \?:                     Operators in Conditionals.
                                                            (line  9068)
* copy mode, and \? <1>:                 Diversions.        (line 11021)
* copy mode, and \a:                     Leaders.           (line  6603)
* copy mode, and \t:                     Tabs and Fields.   (line  6427)
* copy mode, and \V:                     I/O.               (line 11669)
* copying environment (evc):             Environments.      (line 11309)
* correction between oblique and upright glyph (\/, \,): Ligatures and Kerning.
                                                            (line  8286)
* correction between upright and oblique glyph (\/, \,): Ligatures and Kerning.
                                                            (line  8296)
* correction, italic (\/):               Ligatures and Kerning.
                                                            (line  8286)
* correction, left italic (\,):          Ligatures and Kerning.
                                                            (line  8296)
* cover page in [ms], example markup:    ms Document Description Macros.
                                                            (line  2166)
* cp request, and glyph definitions:     Using Symbols.     (line  7910)
* cq glyph, at end of sentence:          Sentences.         (line  3630)
* cq glyph, at end of sentence <1>:      Using Symbols.     (line  7854)
* creating alias for register (aln):     Setting Registers. (line  5282)
* creating alias, for diversion (als):   Strings.           (line  8924)
* creating alias, for macro (als):       Strings.           (line  8924)
* creating alias, for string (als):      Strings.           (line  8924)
* creating new characters (char):        Using Symbols.     (line  7910)
* credits:                               Credits.           (line   450)
* cs request, and font styles:           Font Families.     (line  7419)
* cs request, and font translations:     Changing Fonts.    (line  7323)
* cs request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12568)
* cs request, with fractional type sizes: Using Fractional Type Sizes.
                                                            (line  8554)
* CSTR #54 errata:                       Built-in Registers.
                                                            (line  5516)
* CSTR #54 errata <1>:                   Line Layout.       (line  6893)
* CSTR #54 errata <2>:                   Page Control.      (line  7159)
* CSTR #54 errata <3>:                   Artificial Fonts.  (line  8135)
* CSTR #54 errata <4>:                   Page Motions.      (line  9994)
* CSTR #54 erratum, bp request:          Page Control.      (line  7159)
* CSTR #54 erratum, po request:          Line Layout.       (line  6893)
* CSTR #54 erratum, sb register:         Page Motions.      (line  9994)
* CSTR #54 erratum, st register:         Page Motions.      (line  9994)
* CSTR #54 erratum, yr register:         Built-in Registers.
                                                            (line  5516)
* CSTR #54 erratum, \S escape:           Artificial Fonts.  (line  8135)
* current directory:                     Macro Directories. (line   917)
* current input file name register (.F): Built-in Registers.
                                                            (line  5485)
* current page number (%):               Page Control.      (line  7173)
* current time, hours (hours):           Built-in Registers.
                                                            (line  5502)
* current time, minutes (minutes):       Built-in Registers.
                                                            (line  5499)
* current time, seconds (seconds):       Built-in Registers.
                                                            (line  5496)
* da request, and dn (dl):               Diversions.        (line 10973)
* da request, and warnings:              Warnings.          (line 12255)
* da request, and warnings <1>:          Warnings.          (line 12260)
* date, day of the month register (dy):  Built-in Registers.
                                                            (line  5508)
* date, day of the week register (dw):   Built-in Registers.
                                                            (line  5505)
* date, month of the year register (mo): Built-in Registers.
                                                            (line  5511)
* date, year register (year, yr):        Built-in Registers.
                                                            (line  5514)
* day of the month register (dy):        Built-in Registers.
                                                            (line  5508)
* day of the week register (dw):         Built-in Registers.
                                                            (line  5505)
* dd glyph, at end of sentence:          Sentences.         (line  3630)
* dd glyph, at end of sentence <1>:      Using Symbols.     (line  7854)
* de request, and while:                 while.             (line  9297)
* de, de1, dei requests, and warnings:   Warnings.          (line 12260)
* debugging:                             Debugging.         (line 12036)
* debugging page location traps:         Page Location Traps.
                                                            (line 10444)
* decimal point, as delimiter:           Delimiters.        (line  5050)
* decrementation, automatic, of a register: Auto-increment. (line  5323)
* default color:                         Colors.            (line  8670)
* default tab stops:                     Tabs and Fields.   (line  6435)
* default units:                         Default Units.     (line  4286)
* deferred output:                       Deferring Output.  (line 10328)
* defining character (char):             Using Symbols.     (line  7910)
* defining character class (class):      Character Classes. (line  7993)
* defining fallback character (fchar, fschar, schar): Using Symbols.
                                                            (line  7910)
* defining glyph (char):                 Using Symbols.     (line  7910)
* defining symbol (char):                Using Symbols.     (line  7910)
* delayed text:                          Footnotes and Annotations.
                                                            (line  1288)
* delimited arguments, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12460)
* delimiters, for escape sequence arguments: Delimiters.    (line  5021)
* delimiting character, for fields (fc): Fields.            (line  6640)
* delimiting escape sequence arguments:  Delimiters.        (line  5021)
* depth, interpolation:                  Calling Macros.    (line  4889)
* depth, of last glyph (.cdp):           Environments.      (line 11335)
* DESC file format:                      DESC File Format.  (line 13395)
* DESC file, and font mounting:          Font Positions.    (line  7511)
* DESC file, and use_charnames_in_special keyword: Postprocessor Access.
                                                            (line 11705)
* device description files, comments:    DESC File Format.  (line 13402)
* device request, and copy mode:         Postprocessor Access.
                                                            (line 11692)
* device resolution:                     Page Geometry.     (line  4137)
* device resolution <1>:                 DESC File Format.  (line 13476)
* device resolution, obtaining in the formatter: Measurements.
                                                            (line  4208)
* devices for output:                    Output Device Intro.
                                                            (line   369)
* dg glyph, at end of sentence:          Sentences.         (line  3630)
* dg glyph, at end of sentence <1>:      Using Symbols.     (line  7854)
* di request, and warnings:              Warnings.          (line 12255)
* di request, and warnings <1>:          Warnings.          (line 12260)
* differences in implementation:         Implementation Differences.
                                                            (line 12350)
* digit-width space (\0):                Page Motions.      (line  9968)
* digits, as delimiters:                 Delimiters.        (line  5050)
* dimensions, line:                      Line Layout.       (line  6826)
* directories for fonts:                 Font Directories.  (line   937)
* directories for macros:                Macro Directories. (line   902)
* directory, current:                    Macro Directories. (line   917)
* directory, for tmac files:             Macro Directories. (line   907)
* directory, home:                       Macro Directories. (line   920)
* directory, platform-specific:          Macro Directories. (line   922)
* directory, site-specific:              Macro Directories. (line   922)
* directory, site-specific <1>:          Font Directories.  (line   960)
* disabling hyphenation (\%):            Manipulating Hyphenation.
                                                            (line  5937)
* disabling \ (eo):                      Using Escape Sequences.
                                                            (line  4973)
* discardable horizontal space:          Manipulating Filling and Adjustment.
                                                            (line  5863)
* discarded space in traps:              Manipulating Spacing.
                                                            (line  6318)
* displays:                              Displays.          (line  1258)
* displays [ms]:                         ms keeps and displays.
                                                            (line  2765)
* displays, and footnotes [ms]:          ms Footnotes.      (line  2925)
* distance to next vertical position trap register (.t): Page Location Traps.
                                                            (line 10488)
* diversion:                             Deferring Output.  (line 10328)
* diversion name register (.z):          Diversions.        (line 10947)
* diversion trap, setting (dt):          Diversion Traps.   (line 10611)
* diversion traps:                       Diversion Traps.   (line 10610)
* diversion, appending (da):             Diversions.        (line 10888)
* diversion, beginning (di):             Diversions.        (line 10888)
* diversion, creating alias for (als):   Strings.           (line  8924)
* diversion, ending (di):                Diversions.        (line 10888)
* diversion, nested:                     Diversions.        (line 10947)
* diversion, removing (rm):              Strings.           (line  8919)
* diversion, removing alias for (rm):    Strings.           (line  8959)
* diversion, renaming (rn):              Strings.           (line  8916)
* diversion, stripping final newline:    Punning Names.     (line 11193)
* diversion, top-level:                  Diversions.        (line 10874)
* diversion, top-level, and bp:          Page Control.      (line  7170)
* diversion, top-level, and \!:          Diversions.        (line 11045)
* diversion, top-level, and \?:          Diversions.        (line 11050)
* diversion, unformatting (asciify):     Diversions.        (line 11067)
* diversion, vertical position in, register (.d): Diversions.
                                                            (line 10947)
* diversions:                            Diversions.        (line 10862)
* diversions <1>:                        Punning Names.     (line 11104)
* diversions, and traps:                 Page Location Traps.
                                                            (line 10590)
* diversions, shared name space with macros and strings: Identifiers.
                                                            (line  4632)
* division, truncating:                  Numeric Expressions.
                                                            (line  4327)
* dl register, and da (boxa):            Diversions.        (line 10973)
* dn register, and da (boxa):            Diversions.        (line 10973)
* document description macros, [ms]:     ms Document Description Macros.
                                                            (line  2102)
* documents, multi-file:                 Debugging.         (line 12059)
* documents, structuring the source of:  Invoking Requests. (line  4763)
* dot, as delimiter:                     Delimiters.        (line  5050)
* double quote, embedding in a macro argument: Calling Macros.
                                                            (line  4835)
* double quotes, trailing, in strings:   Strings.           (line  8796)
* double-spacing (ls):                   Basics.            (line  1136)
* double-spacing (ls) <1>:               Manipulating Spacing.
                                                            (line  6351)
* double-spacing (vs, pvs):              Changing the Vertical Spacing.
                                                            (line  8525)
* down-casing a string (stringdown):     Strings.           (line  8897)
* drawing a circle (\D'c ...'):          Drawing Requests.  (line 10181)
* drawing a line (\D'l ...'):            Drawing Requests.  (line 10152)
* drawing a polygon (\D'p ...'):         Drawing Requests.  (line 10230)
* drawing a solid circle (\D'C ...'):    Drawing Requests.  (line 10186)
* drawing a solid ellipse (\D'E ...'):   Drawing Requests.  (line 10196)
* drawing a solid polygon (\D'P ...'):   Drawing Requests.  (line 10239)
* drawing a spline (\D'~ ...'):          Drawing Requests.  (line 10208)
* drawing an arc (\D'a ...'):            Drawing Requests.  (line 10200)
* drawing an ellipse (\D'e ...'):        Drawing Requests.  (line 10190)
* drawing horizontal lines (\l):         Drawing Requests.  (line 10092)
* drawing position:                      Page Geometry.     (line  4149)
* drawing position, vertical (nl):       Page Control.      (line  7224)
* drawing requests:                      Drawing Requests.  (line 10082)
* drawing vertical lines (\L):           Drawing Requests.  (line 10123)
* ds request, and comments:              Strings.           (line  8775)
* ds request, and double quotes:         Strings.           (line  8796)
* ds request, and leading spaces:        Strings.           (line  8796)
* ds, ds1 requests, and comments:        Comments.          (line  5099)
* ds, ds1 requests, and warnings:        Warnings.          (line 12260)
* dummy character (\&):                  Dummy Characters.  (line  8315)
* dummy character (\&), as control character suppressor: Requests and Macros.
                                                            (line  3787)
* dummy character (\&), effect on kerning: Ligatures and Kerning.
                                                            (line  8253)
* dummy character (\&), effect on \l escape: Drawing Requests.
                                                            (line 10107)
* dummy character, transparent (\)):     Dummy Characters.  (line  8363)
* dummy environment, used by \w escape sequence: Page Motions.
                                                            (line  9988)
* dumping environments (pev):            Debugging.         (line 12107)
* dumping page location traps (ptr):     Debugging.         (line 12121)
* dumping registers (pnr):               Debugging.         (line 12117)
* dumping symbol table (pm):             Debugging.         (line 12111)
* EBCDIC, input encoding:                Input Encodings.   (line  3953)
* EBCDIC, output encoding:               Groff Options.     (line   759)
* el request, and warnings:              Warnings.          (line 12232)
* ellipse, drawing (\D'e ...'):          Drawing Requests.  (line 10190)
* ellipse, solid, drawing (\D'E ...'):   Drawing Requests.  (line 10196)
* em glyph, and cflags:                  Using Symbols.     (line  7837)
* em scaling unit (m):                   Measurements.      (line  4245)
* embolding of special fonts:            Artificial Fonts.  (line  8189)
* empty line:                            Breaking.          (line  3716)
* empty line (sp):                       Basics.            (line  1145)
* en scaling unit (n):                   Measurements.      (line  4249)
* enabling vertical position traps (vpt): Vertical Position Traps.
                                                            (line 10397)
* encoding, input, code page 1047:       Input Encodings.   (line  3953)
* encoding, input, EBCDIC:               Input Encodings.   (line  3953)
* encoding, input, Latin-1 (ISO 8859-1): Input Encodings.   (line  3958)
* encoding, input, Latin-2 (ISO 8859-2): Input Encodings.   (line  3971)
* encoding, input, Latin-5 (ISO 8859-9): Input Encodings.   (line  3977)
* encoding, input, Latin-9 (ISO 8859-15): Input Encodings.  (line  3982)
* encoding, output, ASCII:               Groff Options.     (line   747)
* encoding, output, code page 1047:      Groff Options.     (line   759)
* encoding, output, EBCDIC:              Groff Options.     (line   759)
* encoding, output, ISO 646:             Groff Options.     (line   747)
* encoding, output, Latin-1 (ISO 8859-1): Groff Options.    (line   751)
* encoding, output, UTF-8:               Groff Options.     (line   755)
* end of conditional block (\}):         Conditional Blocks.
                                                            (line  9197)
* end-of-input macro (em):               End-of-input Traps.
                                                            (line 10751)
* end-of-input trap, setting (em):       End-of-input Traps.
                                                            (line 10751)
* end-of-input traps:                    End-of-input Traps.
                                                            (line 10750)
* end-of-sentence characters:            Sentences.         (line  3585)
* end-of-sentence characters <1>:        Using Symbols.     (line  7825)
* end-of-sentence transparent characters: Sentences.        (line  3630)
* ending diversion (di):                 Diversions.        (line 10888)
* environment:                           Deferring Output.  (line 10328)
* environment availability and naming, incompatibilities with: Other Differences.
                                                            (line 12542)
* environment number/name register (.ev): Environments.     (line 11256)
* environment variables:                 Environment.       (line   831)
* environment, copying (evc):            Environments.      (line 11309)
* environment, dimensions of last glyph (.w, .cht, .cdp, .csk): Environments.
                                                            (line 11335)
* environment, dummy, used by \w escape sequence: Page Motions.
                                                            (line  9988)
* environment, previous line length (.n): Environments.     (line 11350)
* environment, switching (ev):           Environments.      (line 11256)
* environments:                          Environments.      (line 11215)
* environments, dumping (pev):           Debugging.         (line 12107)
* equality operator:                     Numeric Expressions.
                                                            (line  4384)
* equation example [ms]:                 ms Insertions.     (line  2870)
* equations [ms]:                        ms Insertions.     (line  2811)
* escape character, changing (ec):       Using Escape Sequences.
                                                            (line  4978)
* escape character, formatting (\e):     Using Escape Sequences.
                                                            (line  4958)
* escape character, while defining glyph: Using Symbols.    (line  7910)
* escape sequence:                       Formatter Instructions.
                                                            (line  4678)
* escape sequence argument delimiters:   Delimiters.        (line  5021)
* escape sequences:                      Using Escape Sequences.
                                                            (line  4902)
* escape sequences, brace (\{, \}):      Conditional Blocks.
                                                            (line  9197)
* escaping newline characters, in strings: Strings.         (line  8805)
* ex request, use in debugging:          Debugging.         (line 12089)
* ex request, used with nx and rd:       I/O.               (line 11539)
* example markup, bulleted list [ms]:    Lists in ms.       (line  2558)
* example markup, cover page in [ms]:    ms Document Description Macros.
                                                            (line  2166)
* example markup, glossary-style list [ms]: Lists in ms.    (line  2605)
* example markup, numbered list [ms]:    Lists in ms.       (line  2577)
* examples of invocation:                Invocation Examples.
                                                            (line  1003)
* exiting (ex):                          Debugging.         (line 12089)
* expansion of strings (\*):             Strings.           (line  8754)
* explicit hyphen (\%):                  Manipulating Hyphenation.
                                                            (line  6224)
* explicit hyphenation:                  Manipulating Hyphenation.
                                                            (line  5893)
* expression, limitation of logical not in: Numeric Expressions.
                                                            (line  4394)
* expression, order of evaluation:       Numeric Expressions.
                                                            (line  4414)
* expressions, and register format:      Assigning Register Formats.
                                                            (line  5443)
* expressions, and space characters:     Numeric Expressions.
                                                            (line  4506)
* expressions, conditional:              Operators in Conditionals.
                                                            (line  8979)
* expressions, numeric:                  Numeric Expressions.
                                                            (line  4316)
* extra post-vertical line space (\x):   Changing the Vertical Spacing.
                                                            (line  8518)
* extra post-vertical line space register (.a): Manipulating Spacing.
                                                            (line  6378)
* extra pre-vertical line space (\x):    Changing the Vertical Spacing.
                                                            (line  8510)
* extra spaces between words:            Adjustment.        (line  3742)
* extreme values representable with Roman numerals: Assigning Register Formats.
                                                            (line  5424)
* extremum operators (>?, <?):           Numeric Expressions.
                                                            (line  4375)
* f scaling unit:                        Colors.            (line  8662)
* factor, zoom, of a font (fzoom):       Changing Fonts.    (line  7338)
* fallback character, defining (fchar, fschar, schar): Using Symbols.
                                                            (line  7910)
* fallback glyph, removing definition (rchar, rfschar): Using Symbols.
                                                            (line  7967)
* fam request, and changing fonts:       Changing Fonts.    (line  7279)
* fam request, and font positions:       Font Positions.    (line  7545)
* families, font:                        Font Families.     (line  7365)
* features, common:                      Common Features.   (line  1182)
* fi request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* field delimiting character (fc):       Fields.            (line  6640)
* field padding character (fc):          Fields.            (line  6640)
* fields:                                Fields.            (line  6640)
* fields, and tabs:                      Tabs and Fields.   (line  6423)
* figure space (\0):                     Page Motions.      (line  9968)
* figures [ms]:                          ms Insertions.     (line  2811)
* file formats:                          File Formats.      (line 12634)
* file names, breaking (\:):             Manipulating Hyphenation.
                                                            (line  5953)
* file, appending to (opena):            I/O.               (line 11619)
* file, closing (close):                 I/O.               (line 11651)
* file, inclusion (so):                  I/O.               (line 11420)
* file, macro, search path:              Macro Directories. (line   907)
* file, opening (open):                  I/O.               (line 11619)
* file, processing next (nx):            I/O.               (line 11501)
* file, writing to (write, writec):      I/O.               (line 11631)
* files, font:                           Device and Font Description Files.
                                                            (line 13371)
* fill color:                            Colors.            (line  8628)
* fill color name register (.M):         Colors.            (line  8720)
* fill colors, unnamed (\D'F...'):       Drawing Requests.  (line 10288)
* fill mode (fi), enabling:              Manipulating Filling and Adjustment.
                                                            (line  5637)
* fill mode, and \c:                     Line Continuation. (line  7028)
* fill mode, disabling:                  Manipulating Filling and Adjustment.
                                                            (line  5644)
* filling:                               Filling.           (line  3547)
* filling and adjustment, manipulating:  Manipulating Filling and Adjustment.
                                                            (line  5577)
* filling of output, disabling (nf):     Manipulating Filling and Adjustment.
                                                            (line  5644)
* filling of output, enabling (fi):      Manipulating Filling and Adjustment.
                                                            (line  5637)
* filling, and break warnings:           Warnings.          (line 12221)
* filling, and inter-sentence space:     Manipulating Filling and Adjustment.
                                                            (line  5853)
* final newline, stripping in diversions: Punning Names.    (line 11193)
* fl request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* floating keep:                         Displays.          (line  1275)
* flush output (fl):                     Debugging.         (line 12126)
* font description file format:          DESC File Format.  (line 13395)
* font description file, format:         Font Description File Format.
                                                            (line 13542)
* font description files, comments:      Font Description File Format.
                                                            (line 13562)
* font directories:                      Font Directories.  (line   937)
* font families:                         Font Families.     (line  7365)
* font family, changing (fam, \F):       Font Families.     (line  7384)
* font file, format:                     Font Description File Format.
                                                            (line 13542)
* font files:                            Device and Font Description Files.
                                                            (line 13371)
* font for underlining (uf):             Artificial Fonts.  (line  8165)
* font height, changing (\H):            Artificial Fonts.  (line  8091)
* font path:                             Font Directories.  (line   945)
* font position register (.f):           Font Positions.    (line  7498)
* font position, changing (\f):          Font Positions.    (line  7545)
* font positions:                        Font Positions.    (line  7477)
* font slant, changing (\S):             Artificial Fonts.  (line  8120)
* font style, changing (sty):            Font Families.     (line  7419)
* font styles:                           Font Families.     (line  7365)
* font translation (ftr):                Changing Fonts.    (line  7323)
* font, magnification (fzoom):           Changing Fonts.    (line  7338)
* font, mounting (fp):                   Font Positions.    (line  7489)
* font, optical size:                    Changing Fonts.    (line  7338)
* font, previous (ft, \f[], \fP):        Changing Fonts.    (line  7291)
* font, zoom factor (fzoom):             Changing Fonts.    (line  7338)
* fonts:                                 Fonts and Symbols. (line  7264)
* fonts <1>:                             Changing Fonts.    (line  7274)
* fonts, artificial:                     Artificial Fonts.  (line  8081)
* fonts, changing (ft, \f):              Changing Fonts.    (line  7279)
* fonts, PostScript:                     Font Families.     (line  7370)
* fonts, searching:                      Font Directories.  (line   937)
* fonts, special:                        Special Fonts.     (line  8051)
* footers:                               Page Layout.       (line  7085)
* footers <1>:                           Page Location Traps.
                                                            (line 10422)
* footers [ms]:                          ms Headers and Footers.
                                                            (line  3019)
* footnote marker [ms]:                  ms Footnotes.      (line  2887)
* footnotes:                             Footnotes and Annotations.
                                                            (line  1284)
* footnotes [ms]:                        ms Footnotes.      (line  2887)
* footnotes, and displays [ms]:          ms Footnotes.      (line  2925)
* footnotes, and keeps [ms]:             ms Footnotes.      (line  2925)
* form letters:                          I/O.               (line 11523)
* format of font description file:       DESC File Format.  (line 13395)
* format of font description files:      Font Description File Format.
                                                            (line 13542)
* format of font files:                  Font Description File Format.
                                                            (line 13542)
* format of register (\g):               Assigning Register Formats.
                                                            (line  5438)
* format, paper:                         Paper Format.      (line   973)
* formats, assigning (af):               Assigning Register Formats.
                                                            (line  5372)
* formats, file:                         File Formats.      (line 12634)
* formatter instructions:                Formatter Instructions.
                                                            (line  4667)
* formatting a backslash glyph (\[rs]):  Using Escape Sequences.
                                                            (line  4961)
* formatting the escape character (\e):  Using Escape Sequences.
                                                            (line  4958)
* formatting the time:                   I/O.               (line 11604)
* fp request, and font translations:     Changing Fonts.    (line  7323)
* fp request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12568)
* fractional point sizes:                Using Fractional Type Sizes.
                                                            (line  8547)
* fractional point sizes <1>:            Other Differences. (line 12546)
* fractional type sizes:                 Using Fractional Type Sizes.
                                                            (line  8547)
* fractional type sizes <1>:             Other Differences. (line 12546)
* fractional type sizes in ms macros:    Differences from AT&T ms.
                                                            (line  3281)
* French spacing:                        Sentences.         (line  3585)
* fspecial request, and font styles:     Font Families.     (line  7419)
* fspecial request, and font translations: Changing Fonts.  (line  7323)
* fspecial request, and glyph search order: Using Symbols.  (line  7574)
* fspecial request, and imitating bold:  Artificial Fonts.  (line  8189)
* ft request, and font translations:     Changing Fonts.    (line  7323)
* full-service macro package:            Major Macro Packages.
                                                            (line  1367)
* geometry, page:                        Page Geometry.     (line  4130)
* GGL (groff glyph list):                Using Symbols.     (line  7649)
* GGL (groff glyph list) <1>:            Character Classes. (line  8014)
* glossary-style list, example markup [ms]: Lists in ms.    (line  2605)
* glyph:                                 Using Symbols.     (line  7566)
* glyph for line drawing:                Drawing Requests.  (line 10123)
* glyph names, composite:                Using Symbols.     (line  7649)
* glyph pile (\b):                       Drawing Requests.  (line 10304)
* glyph properties (cflags):             Using Symbols.     (line  7808)
* glyph, box rule (\[br]):               Drawing Requests.  (line 10123)
* glyph, constant space:                 Artificial Fonts.  (line  8200)
* glyph, defining (char):                Using Symbols.     (line  7910)
* glyph, distinguished from character:   Using Symbols.     (line  7566)
* glyph, for line drawing:               Drawing Requests.  (line 10103)
* glyph, for margins (mc):               Miscellaneous.     (line 11849)
* glyph, last, dimensions (.w, .cht, .cdp, .csk): Environments.
                                                            (line 11335)
* glyph, leader repetition (lc):         Leaders.           (line  6606)
* glyph, numbered (\N):                  Character Translations.
                                                            (line  6689)
* glyph, numbered (\N) <1>:              Using Symbols.     (line  7770)
* glyph, removing definition (rchar, rfschar): Using Symbols.
                                                            (line  7967)
* glyph, soft hyphen (hy):               Manipulating Hyphenation.
                                                            (line  5977)
* glyph, tab repetition (tc):            Tabs and Fields.   (line  6544)
* glyph, underscore (\[ru]):             Drawing Requests.  (line 10103)
* glyphs, available, list (groff_char(7) man page): Using Symbols.
                                                            (line  7637)
* glyphs, output, and input characters, compatibility with AT&T troff: Other Differences.
                                                            (line 12568)
* glyphs, overstriking (\o):             Page Motions.      (line 10051)
* glyphs, unnamed:                       Using Symbols.     (line  7780)
* glyphs, unnamed, accessing with \N:    Font Description File Format.
                                                            (line 13609)
* GNU troff, identification register (.g): Built-in Registers.
                                                            (line  5556)
* GNU troff, PID register ($$):          Built-in Registers.
                                                            (line  5552)
* GNU troff, process ID register ($$):   Built-in Registers.
                                                            (line  5552)
* GNU-specific register (.g):            Built-in Registers.
                                                            (line  5556)
* gray shading (\D'f ...'):              Drawing Requests.  (line 10213)
* greater than (or equal to) operator:   Numeric Expressions.
                                                            (line  4384)
* groff capabilities:                    groff Capabilities.
                                                            (line   279)
* groff glyph list (GGL):                Using Symbols.     (line  7649)
* groff glyph list (GGL) <1>:            Character Classes. (line  8014)
* groff invocation:                      Invoking groff.    (line   458)
* groff, and pi request:                 I/O.               (line 11577)
* groff--what is it?:                    What Is groff?.    (line   262)
* GROFF_BIN_PATH, environment variable:  Environment.       (line   835)
* GROFF_COMMAND_PREFIX, environment variable: Environment.  (line   839)
* GROFF_ENCODING, environment variable:  Environment.       (line   850)
* GROFF_FONT_PATH, environment variable: Environment.       (line   859)
* GROFF_FONT_PATH, environment variable <1>: Font Directories.
                                                            (line   957)
* GROFF_TMAC_PATH, environment variable: Environment.       (line   866)
* GROFF_TMAC_PATH, environment variable <1>: Macro Directories.
                                                            (line   914)
* GROFF_TMPDIR, environment variable:    Environment.       (line   873)
* GROFF_TYPESETTER, environment variable: Environment.      (line   881)
* grohtml, the program:                  Groff Options.     (line   770)
* gtroff, interactive use:               Debugging.         (line 12126)
* gtroff, output:                        gtroff Output.     (line 12640)
* gtroff, reference:                     GNU troff Reference.
                                                            (line  3515)
* hair space (\^):                       Page Motions.      (line  9962)
* hcode request, and glyph definitions:  Using Symbols.     (line  7910)
* headers:                               Page Layout.       (line  7085)
* headers <1>:                           Page Location Traps.
                                                            (line 10422)
* headers [ms]:                          ms Headers and Footers.
                                                            (line  3019)
* height, font, changing (\H):           Artificial Fonts.  (line  8091)
* height, of last glyph (.cht):          Environments.      (line 11335)
* high-water mark register (.h):         Diversions.        (line 10954)
* home directory:                        Macro Directories. (line   920)
* horizontal discardable space:          Manipulating Filling and Adjustment.
                                                            (line  5863)
* horizontal input line position register (hp): Page Motions.
                                                            (line 10044)
* horizontal input line position, saving (\k): Page Motions.
                                                            (line 10038)
* horizontal line, drawing (\l):         Drawing Requests.  (line 10092)
* horizontal motion (\h):                Page Motions.      (line  9936)
* horizontal motion quantum:             DESC File Format.  (line 13415)
* horizontal motion quantum register (.H): Motion Quanta.   (line  4267)
* horizontal output line position register (.k): Page Motions.
                                                            (line 10047)
* horizontal resolution:                 DESC File Format.  (line 13415)
* horizontal resolution register (.H):   Motion Quanta.     (line  4267)
* horizontal space (\h):                 Page Motions.      (line  9936)
* horizontal space, unformatting:        Punning Names.     (line 11193)
* horizontal tab character:              Tabs and Leaders.  (line  3752)
* hours, current time (hours):           Built-in Registers.
                                                            (line  5502)
* hpf request, and hyphenation language: Manipulating Hyphenation.
                                                            (line  6210)
* hw request, and hy restrictions:       Manipulating Hyphenation.
                                                            (line  5914)
* hw request, and hyphenation language:  Manipulating Hyphenation.
                                                            (line  6210)
* hy glyph, and cflags:                  Using Symbols.     (line  7837)
* hyphen, explicit (\%):                 Manipulating Hyphenation.
                                                            (line  6224)
* hyphenated lines, consecutive (hlm):   Manipulating Hyphenation.
                                                            (line  6224)
* hyphenating characters:                Using Symbols.     (line  7830)
* hyphenation:                           Hyphenation.       (line  3674)
* hyphenation character (\%):            Manipulating Hyphenation.
                                                            (line  5937)
* hyphenation code (hcode):              Manipulating Hyphenation.
                                                            (line  6175)
* hyphenation consecutive line count register (.hlc): Manipulating Hyphenation.
                                                            (line  6231)
* hyphenation consecutive line limit register (.hlm): Manipulating Hyphenation.
                                                            (line  6231)
* hyphenation exceptions:                Manipulating Hyphenation.
                                                            (line  5900)
* hyphenation language register (.hla):  Manipulating Hyphenation.
                                                            (line  6217)
* hyphenation margin (hym):              Manipulating Hyphenation.
                                                            (line  6237)
* hyphenation margin register (.hym):    Manipulating Hyphenation.
                                                            (line  6247)
* hyphenation mode register (.hy):       Manipulating Hyphenation.
                                                            (line  6001)
* hyphenation parameters, automatic:     Manipulating Hyphenation.
                                                            (line  5988)
* hyphenation pattern files:             Manipulating Hyphenation.
                                                            (line  6063)
* hyphenation patterns (hpf):            Manipulating Hyphenation.
                                                            (line  6117)
* hyphenation space (hys):               Manipulating Hyphenation.
                                                            (line  6252)
* hyphenation space adjustment threshold: Manipulating Hyphenation.
                                                            (line  6252)
* hyphenation space adjustment threshold register (.hys): Manipulating Hyphenation.
                                                            (line  6263)
* hyphenation, automatic:                Manipulating Hyphenation.
                                                            (line  5886)
* hyphenation, disabling (\%):           Manipulating Hyphenation.
                                                            (line  5937)
* hyphenation, explicit:                 Manipulating Hyphenation.
                                                            (line  5893)
* hyphenation, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12519)
* hyphenation, manipulating:             Manipulating Hyphenation.
                                                            (line  5886)
* hyphenation, manual:                   Manipulating Hyphenation.
                                                            (line  5893)
* i scaling unit:                        Measurements.      (line  4217)
* i/o:                                   I/O.               (line 11416)
* IBM code page 1047 input encoding:     Input Encodings.   (line  3953)
* IBM code page 1047 output encoding:    Groff Options.     (line   759)
* identifiers:                           Identifiers.       (line  4538)
* identifiers, undefined:                Identifiers.       (line  4620)
* ie request, and font translations:     Changing Fonts.    (line  7323)
* ie request, and warnings:              Warnings.          (line 12232)
* ie request, operators to use with:     Operators in Conditionals.
                                                            (line  8979)
* if request, and font translations:     Changing Fonts.    (line  7323)
* if request, and the ! operator:        Numeric Expressions.
                                                            (line  4333)
* if request, operators to use with:     Operators in Conditionals.
                                                            (line  8979)
* if-else:                               if-else.           (line  9163)
* if-then:                               if-then.           (line  9122)
* imitating boldface (bd):               Artificial Fonts.  (line  8172)
* implementation differences:            Implementation Differences.
                                                            (line 12350)
* implicit line break:                   Breaking.          (line  3692)
* in request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* in request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* inch scaling unit (i):                 Measurements.      (line  4217)
* including a file (so):                 I/O.               (line 11420)
* incompatibilities with AT&T troff:     Implementation Differences.
                                                            (line 12350)
* increment value without changing the register: Auto-increment.
                                                            (line  5362)
* incrementation, automatic, of a register: Auto-increment. (line  5323)
* indentation (in):                      Line Layout.       (line  6845)
* indentation, of roff source code:      Invoking Requests. (line  4763)
* index, in macro package:               Indices.           (line  1309)
* indicator, scaling:                    Measurements.      (line  4200)
* indirect assignments:                  Interpolating Registers.
                                                            (line  5299)
* input and output requests:             I/O.               (line 11416)
* input characters and output glyphs, compatibility with AT&T troff: Other Differences.
                                                            (line 12568)
* input characters, invalid:             Identifiers.       (line  4545)
* input conventions:                     Input Conventions. (line  4019)
* input encoding, code page 1047:        Input Encodings.   (line  3953)
* input encoding, EBCDIC:                Input Encodings.   (line  3953)
* input encoding, Latin-1 (ISO 8859-1):  Input Encodings.   (line  3958)
* input encoding, Latin-2 (ISO 8859-2):  Input Encodings.   (line  3971)
* input encoding, Latin-5 (ISO 8859-9):  Input Encodings.   (line  3977)
* input encoding, Latin-9 (ISO 8859-15): Input Encodings.   (line  3982)
* input file name, current, register (.F): Built-in Registers.
                                                            (line  5485)
* input level:                           Calling Macros.    (line  4889)
* input level in delimited arguments:    Compatibility Mode.
                                                            (line 12460)
* input line continuation (\<RET>):      Line Continuation. (line  6993)
* input line number register (.c, c.):   Built-in Registers.
                                                            (line  5536)
* input line number, setting (lf):       Debugging.         (line 12059)
* input line position, horizontal, saving (\k): Page Motions.
                                                            (line 10038)
* input line trap, clearing (it, itc):   Input Line Traps.  (line 10624)
* input line trap, setting (it, itc):    Input Line Traps.  (line 10624)
* input line traps:                      Input Line Traps.  (line 10622)
* input line traps and interrupted lines (itc): Input Line Traps.
                                                            (line 10648)
* input line, horizontal position, register (hp): Page Motions.
                                                            (line 10044)
* input line, productive:                Manipulating Filling and Adjustment.
                                                            (line  5749)
* input stack, backtrace (backtrace):    Debugging.         (line 12135)
* input stack, setting limit:            Debugging.         (line 12156)
* input token:                           Gtroff Internals.  (line 11928)
* input, 8-bit:                          Font Description File Format.
                                                            (line 13609)
* input, standard, reading from (rd):    I/O.               (line 11506)
* inserting horizontal space (\h):       Page Motions.      (line  9936)
* installation:                          Installation.      (line   378)
* instructing the formatter:             Formatter Instructions.
                                                            (line  4667)
* inter-sentence space size register (.sss): Manipulating Filling and Adjustment.
                                                            (line  5832)
* inter-sentence space, additional:      Manipulating Filling and Adjustment.
                                                            (line  5843)
* inter-word spacing, minimal:           Manipulating Filling and Adjustment.
                                                            (line  5839)
* interactive use of gtroff:             Debugging.         (line 12126)
* intercepting requests:                 Control Characters.
                                                            (line  4727)
* intermediate output:                   gtroff Output.     (line 12650)
* interpolating registers (\n):          Interpolating Registers.
                                                            (line  5294)
* interpolation:                         Requests and Macros.
                                                            (line  3807)
* interpolation depth:                   Calling Macros.    (line  4889)
* interpolation depth in delimited arguments: Compatibility Mode.
                                                            (line 12460)
* interpolation of strings (\*):         Strings.           (line  8754)
* interpretation mode:                   Copy Mode.         (line  9662)
* interrupted line:                      Line Continuation. (line  7022)
* interrupted line register (.int):      Line Continuation. (line  7047)
* interrupted lines and input line traps (itc): Input Line Traps.
                                                            (line 10648)
* introduction:                          Introduction.      (line   220)
* invalid characters for trf request:    I/O.               (line 11490)
* invalid input characters:              Identifiers.       (line  4545)
* invocation examples:                   Invocation Examples.
                                                            (line  1003)
* invoking groff:                        Invoking groff.    (line   458)
* invoking requests:                     Invoking Requests. (line  4741)
* ISO 646, output encoding:              Groff Options.     (line   747)
* ISO 8859-1 (Latin-1), input encoding:  Input Encodings.   (line  3958)
* ISO 8859-1 (Latin-1), output encoding: Groff Options.     (line   751)
* ISO 8859-15 (Latin-9), input encoding: Input Encodings.   (line  3982)
* ISO 8859-2 (Latin-2), input encoding:  Input Encodings.   (line  3971)
* ISO 8859-9 (Latin-5), input encoding:  Input Encodings.   (line  3977)
* italic correction (\/):                Ligatures and Kerning.
                                                            (line  8286)
* justifying text:                       Manipulating Filling and Adjustment.
                                                            (line  5577)
* justifying text (rj):                  Manipulating Filling and Adjustment.
                                                            (line  5822)
* keep:                                  Displays.          (line  1270)
* keep, floating:                        Displays.          (line  1275)
* keeps [ms]:                            ms keeps and displays.
                                                            (line  2711)
* keeps, and footnotes [ms]:             ms Footnotes.      (line  2925)
* kerning and ligatures:                 Ligatures and Kerning.
                                                            (line  8212)
* kerning enabled register (.kern):      Ligatures and Kerning.
                                                            (line  8248)
* kerning, activating (kern):            Ligatures and Kerning.
                                                            (line  8248)
* kerning, track:                        Ligatures and Kerning.
                                                            (line  8259)
* landscape page orientation:            Paper Format.      (line   973)
* language [ms]:                         ms language and localization.
                                                            (line  2970)
* last glyph, dimensions (.w, .cht, .cdp, .csk): Environments.
                                                            (line 11335)
* last-requested point size registers (.psr, .sr): Using Fractional Type Sizes.
                                                            (line  8588)
* last-requested type size registers (.psr, .sr): Using Fractional Type Sizes.
                                                            (line  8588)
* Latin-1 (ISO 8859-1), input encoding:  Input Encodings.   (line  3958)
* Latin-1 (ISO 8859-1), output encoding: Groff Options.     (line   751)
* Latin-2 (ISO 8859-2), input encoding:  Input Encodings.   (line  3971)
* Latin-5 (ISO 8859-9), input encoding:  Input Encodings.   (line  3977)
* Latin-9 (ISO 8859-15), input encoding: Input Encodings.   (line  3982)
* layout, line:                          Line Layout.       (line  6826)
* layout, page:                          Page Layout.       (line  7061)
* lc request, and glyph definitions:     Using Symbols.     (line  7910)
* leader character:                      Tabs and Leaders.  (line  3752)
* leader character <1>:                  Leaders.           (line  6597)
* leader character, and translations:    Character Translations.
                                                            (line  6699)
* leader character, non-interpreted (\a): Leaders.          (line  6603)
* leader repetition character (lc):      Leaders.           (line  6606)
* leaders:                               Leaders.           (line  6590)
* leading:                               Manipulating Type Size and Vertical Spacing.
                                                            (line  8400)
* leading space macro (lsm):             Breaking.          (line  3724)
* leading space traps:                   Leading Space Traps.
                                                            (line 10732)
* leading spaces:                        Breaking.          (line  3724)
* leading spaces macro (lsm):            Leading Space Traps.
                                                            (line 10735)
* leading spaces with ds:                Strings.           (line  8796)
* left italic correction (\,):           Ligatures and Kerning.
                                                            (line  8296)
* left margin (po):                      Line Layout.       (line  6841)
* length of a string (length):           Strings.           (line  8861)
* length of line (ll):                   Line Layout.       (line  6849)
* length of page (pl):                   Page Layout.       (line  7068)
* length of previous line (.n):          Environments.      (line 11350)
* length of title line (lt):             Page Layout.       (line  7117)
* length request, and copy mode:         Strings.           (line  8861)
* less than (or equal to) operator:      Numeric Expressions.
                                                            (line  4384)
* letters, form:                         I/O.               (line 11523)
* level, input:                          Calling Macros.    (line  4889)
* level, suppression nesting, register:  Suppressing Output.
                                                            (line 11410)
* lf request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12538)
* ligature:                              Using Symbols.     (line  7566)
* ligatures and kerning:                 Ligatures and Kerning.
                                                            (line  8212)
* ligatures enabled register (.lg):      Ligatures and Kerning.
                                                            (line  8230)
* ligatures, activating (lg):            Ligatures and Kerning.
                                                            (line  8230)
* limitations of \b escape sequence:     Drawing Requests.  (line 10312)
* line break:                            Basics.            (line  1102)
* line break <1>:                        Manipulating Filling and Adjustment.
                                                            (line  5577)
* line break (br):                       Basics.            (line  1169)
* line break, output:                    Breaking.          (line  3692)
* line control:                          Line Continuation. (line  6988)
* line dimensions:                       Line Layout.       (line  6826)
* line drawing glyph:                    Drawing Requests.  (line 10103)
* line drawing glyph <1>:                Drawing Requests.  (line 10123)
* line indentation (in):                 Line Layout.       (line  6845)
* line layout:                           Line Layout.       (line  6826)
* line length (ll):                      Line Layout.       (line  6849)
* line length register (.l):             Line Layout.       (line  6977)
* line length, previous (.n):            Environments.      (line 11350)
* line number, input, register (.c, c.): Built-in Registers.
                                                            (line  5536)
* line number, output, register (ln):    Miscellaneous.     (line 11770)
* line numbers, printing (nm):           Miscellaneous.     (line 11740)
* line space, extra post-vertical (\x):  Changing the Vertical Spacing.
                                                            (line  8518)
* line space, extra pre-vertical (\x):   Changing the Vertical Spacing.
                                                            (line  8510)
* line spacing register (.L):            Manipulating Spacing.
                                                            (line  6359)
* line spacing, post-vertical (pvs):     Changing the Vertical Spacing.
                                                            (line  8522)
* line thickness (\D't ...'):            Drawing Requests.  (line 10278)
* line, blank:                           Breaking.          (line  3716)
* line, drawing (\D'l ...'):             Drawing Requests.  (line 10152)
* line, empty (sp):                      Basics.            (line  1145)
* line, horizontal, drawing (\l):        Drawing Requests.  (line 10092)
* line, input, continuation (\<RET>):    Line Continuation. (line  6993)
* line, input, horizontal position, register (hp): Page Motions.
                                                            (line 10044)
* line, input, horizontal position, saving (\k): Page Motions.
                                                            (line 10038)
* line, interrupted:                     Line Continuation. (line  7022)
* line, output, continuation (\c):       Line Continuation. (line  7022)
* line, output, horizontal position, register (.k): Page Motions.
                                                            (line 10047)
* line, productive input:                Manipulating Filling and Adjustment.
                                                            (line  5749)
* line, vertical, drawing (\L):          Drawing Requests.  (line 10123)
* line-tabs mode:                        Tabs and Fields.   (line  6557)
* lines, blank, disabling:               Manipulating Spacing.
                                                            (line  6407)
* lines, centering (ce):                 Basics.            (line  1157)
* lines, centering (ce) <1>:             Manipulating Filling and Adjustment.
                                                            (line  5777)
* lines, consecutive hyphenated (hlm):   Manipulating Hyphenation.
                                                            (line  6224)
* lines, interrupted, and input line traps (itc): Input Line Traps.
                                                            (line 10648)
* list:                                  Displays.          (line  1265)
* list of available glyphs (groff_char(7) man page): Using Symbols.
                                                            (line  7637)
* listing page location traps (ptr):     Debugging.         (line 12121)
* ll request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* localization:                          Manipulating Hyphenation.
                                                            (line  6162)
* localization [ms]:                     ms language and localization.
                                                            (line  2970)
* locating macro files:                  Macro Directories. (line   907)
* locating macro packages:               Macro Directories. (line   907)
* location, vertical, page, marking (mk): Page Motions.     (line  9822)
* location, vertical, page, returning to marked (rt): Page Motions.
                                                            (line  9822)
* logical "and" operator:                Numeric Expressions.
                                                            (line  4390)
* logical "or" operator:                 Numeric Expressions.
                                                            (line  4390)
* logical complementation operator:      Numeric Expressions.
                                                            (line  4394)
* logical conjunction operator:          Numeric Expressions.
                                                            (line  4390)
* logical disjunction operator:          Numeric Expressions.
                                                            (line  4390)
* logical not, limitation in expression: Numeric Expressions.
                                                            (line  4394)
* logical operators:                     Numeric Expressions.
                                                            (line  4390)
* long names:                            Compatibility Mode.
                                                            (line 12366)
* loops and conditionals:                Conditionals and Loops.
                                                            (line  8972)
* lowercasing a string (stringdown):     Strings.           (line  8897)
* ls request, alternative to (pvs):      Changing the Vertical Spacing.
                                                            (line  8534)
* lt request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* m scaling unit:                        Measurements.      (line  4245)
* M scaling unit:                        Measurements.      (line  4255)
* machine units:                         Page Geometry.     (line  4137)
* macro:                                 Requests and Macros.
                                                            (line  3807)
* macro arguments:                       Calling Macros.    (line  4811)
* macro arguments, and compatibility mode: Gtroff Internals.
                                                            (line 12012)
* macro arguments, and tabs:             Invoking Requests. (line  4749)
* macro basics:                          Basics.            (line  1059)
* macro directories:                     Macro Directories. (line   902)
* macro file search path:                Macro Directories. (line   907)
* macro name register (\$0):             Parameters.        (line  9611)
* macro names, starting with [ or ], and refer: Identifiers.
                                                            (line  4587)
* macro package:                         Macro Packages.    (line  3930)
* macro package search path:             Macro Directories. (line   907)
* macro package, auxiliary:              Major Macro Packages.
                                                            (line  1378)
* macro package, full-service:           Major Macro Packages.
                                                            (line  1367)
* macro package, introduction:           Macro Package Intro.
                                                            (line   326)
* macro package, major:                  Major Macro Packages.
                                                            (line  1364)
* macro package, minor:                  Major Macro Packages.
                                                            (line  1378)
* macro package, structuring the source of: Invoking Requests.
                                                            (line  4763)
* macro, appending to (am):              Writing Macros.    (line  9487)
* macro, creating alias for (als):       Strings.           (line  8924)
* macro, end-of-input (em):              End-of-input Traps.
                                                            (line 10751)
* macro, parameters (\$):                Parameters.        (line  9570)
* macro, removing (rm):                  Strings.           (line  8919)
* macro, removing alias for (rm):        Strings.           (line  8959)
* macro, renaming (rn):                  Strings.           (line  8916)
* macros, recursive:                     while.             (line  9313)
* macros, searching:                     Macro Directories. (line   902)
* macros, shared name space with strings and diversions: Identifiers.
                                                            (line  4632)
* macros, tutorial for users:            Tutorial for Macro Users.
                                                            (line  1047)
* macros, writing:                       Writing Macros.    (line  9364)
* magnification of a font (fzoom):       Changing Fonts.    (line  7338)
* major macro package:                   Major Macro Packages.
                                                            (line  1364)
* major quotes:                          Displays.          (line  1262)
* major version number register (.x):    Built-in Registers.
                                                            (line  5540)
* man macros, custom headers and footers: Optional man extensions.
                                                            (line  1403)
* man macros, Ultrix-specific:           Optional man extensions.
                                                            (line  1421)
* man pages:                             man.               (line  1387)
* manipulating filling and adjustment:   Manipulating Filling and Adjustment.
                                                            (line  5577)
* manipulating hyphenation:              Manipulating Hyphenation.
                                                            (line  5886)
* manipulating spacing:                  Manipulating Spacing.
                                                            (line  6294)
* manipulating type size and vertical spacing: Manipulating Type Size and Vertical Spacing.
                                                            (line  8394)
* manual hyphenation:                    Manipulating Hyphenation.
                                                            (line  5893)
* manual pages:                          man.               (line  1387)
* margin for hyphenation (hym):          Manipulating Hyphenation.
                                                            (line  6237)
* margin glyph (mc):                     Miscellaneous.     (line 11849)
* margin, bottom:                        Page Layout.       (line  7074)
* margin, left (po):                     Line Layout.       (line  6841)
* margin, right:                         Line Layout.       (line  6852)
* margin, top:                           Page Layout.       (line  7074)
* mark, high-water, register (.h):       Diversions.        (line 10954)
* marker, footnote [ms]:                 ms Footnotes.      (line  2887)
* marking vertical page location (mk):   Page Motions.      (line  9822)
* maximum operator:                      Numeric Expressions.
                                                            (line  4375)
* maximum value representable with Roman numerals: Assigning Register Formats.
                                                            (line  5424)
* mdoc macros:                           mdoc.              (line  1522)
* me macro package:                      me.                (line  1529)
* measurement units:                     Measurements.      (line  4200)
* measurements:                          Measurements.      (line  4200)
* measurements, specifying safely:       Default Units.     (line  4309)
* minimal inter-word spacing:            Manipulating Filling and Adjustment.
                                                            (line  5839)
* minimum operator:                      Numeric Expressions.
                                                            (line  4375)
* minimum value representable with Roman numerals: Assigning Register Formats.
                                                            (line  5424)
* minor macro package:                   Major Macro Packages.
                                                            (line  1378)
* minor version number register (.y):    Built-in Registers.
                                                            (line  5544)
* minutes, current time (minutes):       Built-in Registers.
                                                            (line  5499)
* mm macro package:                      mm.                (line  1541)
* mode for constant glyph space (cs):    Artificial Fonts.  (line  8200)
* mode, compatibility:                   Compatibility Mode.
                                                            (line 12366)
* mode, compatibility, and parameters:   Gtroff Internals.  (line 12012)
* mode, copy:                            Copy Mode.         (line  9653)
* mode, copy <1>:                        Copy Mode.         (line  9653)
* mode, copy, and cf request:            I/O.               (line 11468)
* mode, copy, and device request:        Postprocessor Access.
                                                            (line 11692)
* mode, copy, and length request:        Strings.           (line  8861)
* mode, copy, and macro parameters:      Parameters.        (line  9570)
* mode, copy, and output request:        Diversions.        (line 11053)
* mode, copy, and trf request:           I/O.               (line 11468)
* mode, copy, and write request:         I/O.               (line 11631)
* mode, copy, and writec request:        I/O.               (line 11631)
* mode, copy, and writem request:        I/O.               (line 11645)
* mode, copy, and \!:                    Diversions.        (line 11021)
* mode, copy, and \?:                    Operators in Conditionals.
                                                            (line  9068)
* mode, copy, and \? <1>:                Diversions.        (line 11021)
* mode, copy, and \a:                    Leaders.           (line  6603)
* mode, copy, and \t:                    Tabs and Fields.   (line  6427)
* mode, copy, and \V:                    I/O.               (line 11669)
* mode, fill (fi), enabling:             Manipulating Filling and Adjustment.
                                                            (line  5637)
* mode, fill, and break warnings:        Warnings.          (line 12221)
* mode, fill, and inter-sentence space:  Manipulating Filling and Adjustment.
                                                            (line  5853)
* mode, fill, and \c:                    Line Continuation. (line  7028)
* mode, fill, disabling:                 Manipulating Filling and Adjustment.
                                                            (line  5644)
* mode, interpretation:                  Copy Mode.         (line  9662)
* mode, line-tabs:                       Tabs and Fields.   (line  6557)
* mode, no-fill:                         Manipulating Filling and Adjustment.
                                                            (line  5644)
* mode, no-fill, and \c:                 Line Continuation. (line  7036)
* mode, no-space (ns):                   Manipulating Spacing.
                                                            (line  6407)
* mode, nroff:                           troff and nroff Modes.
                                                            (line  6784)
* mode, safer:                           Groff Options.     (line   711)
* mode, safer <1>:                       Macro Directories. (line   917)
* mode, safer <2>:                       Built-in Registers.
                                                            (line  5492)
* mode, safer <3>:                       I/O.               (line 11451)
* mode, safer <4>:                       I/O.               (line 11564)
* mode, safer <5>:                       I/O.               (line 11586)
* mode, safer <6>:                       I/O.               (line 11625)
* mode, safer <7>:                       Safer Mode.        (line 12357)
* mode, troff:                           troff and nroff Modes.
                                                            (line  6784)
* mode, unsafe:                          Groff Options.     (line   787)
* mode, unsafe <1>:                      Macro Directories. (line   917)
* mode, unsafe <2>:                      Built-in Registers.
                                                            (line  5492)
* mode, unsafe <3>:                      I/O.               (line 11451)
* mode, unsafe <4>:                      I/O.               (line 11564)
* mode, unsafe <5>:                      I/O.               (line 11586)
* mode, unsafe <6>:                      I/O.               (line 11625)
* modifying requests:                    Control Characters.
                                                            (line  4727)
* modulus:                               Numeric Expressions.
                                                            (line  4327)
* mom macro package:                     mom.               (line  1551)
* month of the year register (mo):       Built-in Registers.
                                                            (line  5511)
* motion operators:                      Numeric Expressions.
                                                            (line  4425)
* motion quanta:                         Motion Quanta.     (line  4260)
* motion quantum, horizontal:            DESC File Format.  (line 13415)
* motion quantum, horizontal, register (.H): Motion Quanta. (line  4267)
* motion quantum, vertical:              DESC File Format.  (line 13523)
* motion, horizontal (\h):               Page Motions.      (line  9936)
* motion, vertical (\v):                 Page Motions.      (line  9892)
* motions, page:                         Page Motions.      (line  9817)
* mounting font (fp):                    Font Positions.    (line  7489)
* ms macros:                             ms.                (line  1573)
* ms macros, accent marks:               ms Legacy Features.
                                                            (line  3371)
* ms macros, body text:                  ms Body Text.      (line  2201)
* ms macros, creating table of contents: ms TOC.            (line  3106)
* ms macros, displays:                   ms keeps and displays.
                                                            (line  2711)
* ms macros, document control settings:  ms Document Control Settings.
                                                            (line  1764)
* ms macros, document description:       ms Document Description Macros.
                                                            (line  2102)
* ms macros, equations:                  ms Insertions.     (line  2811)
* ms macros, figures:                    ms Insertions.     (line  2811)
* ms macros, footers:                    ms Headers and Footers.
                                                            (line  3019)
* ms macros, footnotes:                  ms Footnotes.      (line  2887)
* ms macros, fractional type sizes in:   Differences from AT&T ms.
                                                            (line  3281)
* ms macros, general structure:          ms Document Structure.
                                                            (line  1717)
* ms macros, groff differences from AT&T: Differences from AT&T ms.
                                                            (line  3242)
* ms macros, headers:                    ms Headers and Footers.
                                                            (line  3019)
* ms macros, headings:                   Headings in ms.    (line  2322)
* ms macros, keeps:                      ms keeps and displays.
                                                            (line  2711)
* ms macros, language:                   ms language and localization.
                                                            (line  2970)
* ms macros, lists:                      Lists in ms.       (line  2550)
* ms macros, localization:               ms language and localization.
                                                            (line  2970)
* ms macros, margins:                    ms Margins.        (line  3076)
* ms macros, multiple columns:           ms Multiple Columns.
                                                            (line  3084)
* ms macros, naming conventions:         ms Naming Conventions.
                                                            (line  3486)
* ms macros, nested lists:               Indented regions in ms.
                                                            (line  2681)
* ms macros, obtaining typographical symbols: Typographical symbols in ms.
                                                            (line  2231)
* ms macros, page layout:                ms Page Layout.    (line  3012)
* ms macros, paragraph handling:         Paragraphs in ms.  (line  2247)
* ms macros, references:                 ms Insertions.     (line  2811)
* ms macros, special characters:         ms Legacy Features.
                                                            (line  3371)
* ms macros, strings:                    ms Legacy Features.
                                                            (line  3371)
* ms macros, tables:                     ms Insertions.     (line  2811)
* ms macros, text settings:              Text settings in ms.
                                                            (line  2209)
* multi-file documents:                  Debugging.         (line 12059)
* multi-line strings:                    Strings.           (line  8805)
* multi-page table example [ms]:         ms Insertions.     (line  2853)
* multiple columns [ms]:                 ms Multiple Columns.
                                                            (line  3084)
* multiplication:                        Numeric Expressions.
                                                            (line  4327)
* n scaling unit:                        Measurements.      (line  4249)
* name space, common, of macros, diversions, and strings: Identifiers.
                                                            (line  4632)
* name, background color, register (.M): Colors.            (line  8720)
* name, fill color, register (.M):       Colors.            (line  8720)
* name, stroke color, register (.m):     Colors.            (line  8694)
* named character (\C):                  Using Symbols.     (line  7754)
* names, long:                           Compatibility Mode.
                                                            (line 12366)
* naming conventions, ms macros:         ms Naming Conventions.
                                                            (line  3486)
* ne request, and the .trunc register:   Page Location Traps.
                                                            (line 10556)
* ne request, comparison with sv:        Page Control.      (line  7212)
* negating register values:              Setting Registers. (line  5249)
* negation:                              Numeric Expressions.
                                                            (line  4333)
* nested assignments:                    Interpolating Registers.
                                                            (line  5299)
* nested diversions:                     Diversions.        (line 10947)
* nested lists [ms]:                     Indented regions in ms.
                                                            (line  2681)
* nesting level, suppression, register:  Suppressing Output.
                                                            (line 11410)
* new page (bp):                         Basics.            (line  1143)
* new page (bp) <1>:                     Page Control.      (line  7154)
* newline character, and translations:   Character Translations.
                                                            (line  6699)
* newline character, in strings, escaping: Strings.         (line  8805)
* newline, as delimiter:                 Delimiters.        (line  5036)
* newline, final, stripping in diversions: Punning Names.   (line 11193)
* next file, processing (nx):            I/O.               (line 11501)
* next free font position register (.fp): Font Positions.   (line  7510)
* nf request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* nl register, and .d:                   Diversions.        (line 10947)
* nl register, difference to .h:         Diversions.        (line 10967)
* nm request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* no-break control character ('):        Requests and Macros.
                                                            (line  3780)
* no-break control character, changing (c2): Control Characters.
                                                            (line  4702)
* no-fill mode:                          Manipulating Filling and Adjustment.
                                                            (line  5644)
* no-fill mode, and \c:                  Line Continuation. (line  7036)
* no-space mode (ns):                    Manipulating Spacing.
                                                            (line  6407)
* node, output:                          Gtroff Internals.  (line 11928)
* non-printing break point (\:):         Manipulating Hyphenation.
                                                            (line  5953)
* nr request, and warnings:              Warnings.          (line 12272)
* nr request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* nroff mode:                            troff and nroff Modes.
                                                            (line  6784)
* number of registers register (.R):     Built-in Registers.
                                                            (line  5488)
* number, input line, setting (lf):      Debugging.         (line 12059)
* number, page (pn):                     Page Layout.       (line  7132)
* numbered glyph (\N):                   Character Translations.
                                                            (line  6689)
* numbered glyph (\N) <1>:               Using Symbols.     (line  7770)
* numbered list, example markup [ms]:    Lists in ms.       (line  2577)
* numbers, line, printing (nm):          Miscellaneous.     (line 11740)
* numeral-width space (\0):              Page Motions.      (line  9968)
* numerals, as delimiters:               Delimiters.        (line  5050)
* numerals, Roman:                       Assigning Register Formats.
                                                            (line  5392)
* numeric expression, valid:             Numeric Expressions.
                                                            (line  4490)
* numeric expressions:                   Numeric Expressions.
                                                            (line  4316)
* object creation:                       Writing Macros.    (line  9512)
* offset, page:                          Page Geometry.     (line  4161)
* offset, page (po):                     Line Layout.       (line  6841)
* open request, and safer mode:          Groff Options.     (line   711)
* opena request, and safer mode:         Groff Options.     (line   711)
* opening brace escape sequence (\}):    Conditional Blocks.
                                                            (line  9197)
* opening file (open):                   I/O.               (line 11619)
* operator, scaling:                     Numeric Expressions.
                                                            (line  4358)
* operators, arithmetic:                 Numeric Expressions.
                                                            (line  4327)
* operators, as delimiters:              Delimiters.        (line  5052)
* operators, comparison:                 Numeric Expressions.
                                                            (line  4384)
* operators, extremum (>?, <?):          Numeric Expressions.
                                                            (line  4375)
* operators, logical:                    Numeric Expressions.
                                                            (line  4390)
* operators, motion:                     Numeric Expressions.
                                                            (line  4425)
* operators, unary arithmetic:           Numeric Expressions.
                                                            (line  4333)
* optical size of a font:                Changing Fonts.    (line  7338)
* options:                               Groff Options.     (line   482)
* order of evaluation in expressions:    Numeric Expressions.
                                                            (line  4414)
* orientation, landscape:                Paper Format.      (line   973)
* orphan lines, preventing with ne:      Page Control.      (line  7179)
* os request, and no-space mode:         Page Control.      (line  7217)
* output and input requests:             I/O.               (line 11416)
* output comparison operator:            Operators in Conditionals.
                                                            (line  9032)
* output device name string (.T):        Groff Options.     (line   776)
* output device name string (.T) <1>:    Strings.           (line  8739)
* output device name string (.T), in other implementations: Other Differences.
                                                            (line 12526)
* output device usage register (.T):     Groff Options.     (line   776)
* output device usage register (.T), incompatibility with AT&T troff: Other Differences.
                                                            (line 12533)
* output devices:                        Output Device Intro.
                                                            (line   369)
* output encoding, ASCII:                Groff Options.     (line   747)
* output encoding, code page 1047:       Groff Options.     (line   759)
* output encoding, EBCDIC:               Groff Options.     (line   759)
* output encoding, ISO 646:              Groff Options.     (line   747)
* output encoding, Latin-1 (ISO 8859-1): Groff Options.     (line   751)
* output encoding, UTF-8:                Groff Options.     (line   755)
* output glyphs, and input characters, compatibility with AT&T troff: Other Differences.
                                                            (line 12568)
* output line break:                     Breaking.          (line  3692)
* output line number register (ln):      Miscellaneous.     (line 11770)
* output line, continuation (\c):        Line Continuation. (line  7022)
* output line, horizontal position, register (.k): Page Motions.
                                                            (line 10047)
* output node:                           Gtroff Internals.  (line 11928)
* output request, and copy mode:         Diversions.        (line 11053)
* output request, and \!:                Diversions.        (line 11053)
* output, filling, disablement of (nf):  Manipulating Filling and Adjustment.
                                                            (line  5644)
* output, filling, enablement of (fi):   Manipulating Filling and Adjustment.
                                                            (line  5637)
* output, flush (fl):                    Debugging.         (line 12126)
* output, gtroff:                        gtroff Output.     (line 12640)
* output, intermediate:                  gtroff Output.     (line 12650)
* output, suppressing (\O):              Suppressing Output.
                                                            (line 11357)
* output, transparent (cf, trf):         I/O.               (line 11468)
* output, transparent (\!, \?):          Diversions.        (line 11013)
* output, transparent, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12596)
* output, troff:                         gtroff Output.     (line 12650)
* overlapping characters:                Using Symbols.     (line  7844)
* overstriking glyphs (\o):              Page Motions.      (line 10051)
* p scaling unit:                        Measurements.      (line  4223)
* P scaling unit:                        Measurements.      (line  4227)
* package, macro:                        Macro Packages.    (line  3930)
* package, macro, auxiliary:             Major Macro Packages.
                                                            (line  1378)
* package, macro, full-service:          Major Macro Packages.
                                                            (line  1367)
* package, macro, introduction:          Macro Package Intro.
                                                            (line   326)
* package, macro, major:                 Major Macro Packages.
                                                            (line  1364)
* package, macro, minor:                 Major Macro Packages.
                                                            (line  1378)
* package, macro, search path:           Macro Directories. (line   907)
* package, package, structuring the source of: Invoking Requests.
                                                            (line  4763)
* padding character, for fields (fc):    Fields.            (line  6640)
* page:                                  Page Geometry.     (line  4143)
* page break:                            Page Geometry.     (line  4176)
* page break, conditional (ne):          Page Control.      (line  7179)
* page control:                          Page Control.      (line  7150)
* page ejecting register (.pe):          Page Location Traps.
                                                            (line 10568)
* page footers:                          Page Location Traps.
                                                            (line 10422)
* page headers:                          Page Location Traps.
                                                            (line 10422)
* page layout:                           Page Layout.       (line  7061)
* page layout [ms]:                      ms Page Layout.    (line  3012)
* page length (pl):                      Page Layout.       (line  7068)
* page length register (.p):             Page Layout.       (line  7072)
* page location traps:                   Page Location Traps.
                                                            (line 10409)
* page location traps, debugging:        Page Location Traps.
                                                            (line 10444)
* page location, vertical, marking (mk): Page Motions.      (line  9822)
* page location, vertical, returning to marked (rt): Page Motions.
                                                            (line  9822)
* page motions:                          Page Motions.      (line  9817)
* page number (pn):                      Page Layout.       (line  7132)
* page number character (%):             Page Layout.       (line  7089)
* page number character, changing (pc):  Page Layout.       (line  7141)
* page number register (%):              Page Control.      (line  7173)
* page offset:                           Page Geometry.     (line  4161)
* page offset (po):                      Line Layout.       (line  6841)
* page orientation, landscape:           Paper Format.      (line   973)
* page, geometry of:                     Page Geometry.     (line  4130)
* page, new (bp):                        Page Control.      (line  7154)
* paper format:                          Paper Format.      (line   973)
* paper formats:                         Paper Formats.     (line  1319)
* paper size:                            Paper Format.      (line   973)
* paragraphs:                            Paragraphs.        (line  1193)
* parameter count register (.$):         Parameters.        (line  9548)
* parameters:                            Parameters.        (line  9540)
* parameters, and compatibility mode:    Gtroff Internals.  (line 12012)
* parameters, macro (\$):                Parameters.        (line  9570)
* parentheses:                           Numeric Expressions.
                                                            (line  4414)
* partially collected line:              Manipulating Filling and Adjustment.
                                                            (line  5597)
* path, for font files:                  Font Directories.  (line   945)
* path, for tmac files:                  Macro Directories. (line   907)
* pattern files, for hyphenation:        Manipulating Hyphenation.
                                                            (line  6063)
* patterns for hyphenation (hpf):        Manipulating Hyphenation.
                                                            (line  6117)
* pending output line:                   Manipulating Filling and Adjustment.
                                                            (line  5597)
* pi request, and groff:                 I/O.               (line 11577)
* pi request, and safer mode:            Groff Options.     (line   711)
* pi request, disabled by default:       Safer Mode.        (line 12357)
* pica scaling unit (P):                 Measurements.      (line  4227)
* PID of GNU troff register ($$):        Built-in Registers.
                                                            (line  5552)
* pile, glyph (\b):                      Drawing Requests.  (line 10304)
* pl request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* plain text approximation output register (.A): Groff Options.
                                                            (line   525)
* plain text approximation output register (.A) <1>: Built-in Registers.
                                                            (line  5560)
* planting a trap:                       Traps.             (line 10384)
* platform-specific directory:           Macro Directories. (line   922)
* pm request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12559)
* pn request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* PNG image generation from PostScript:  DESC File Format.  (line 13419)
* po request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* point scaling unit (p):                Measurements.      (line  4223)
* point size registers (.s, .ps):        Changing the Type Size.
                                                            (line  8429)
* point size registers, last-requested (.psr, .sr): Using Fractional Type Sizes.
                                                            (line  8588)
* point sizes, changing (ps, \s):        Changing the Type Size.
                                                            (line  8422)
* point sizes, fractional:               Using Fractional Type Sizes.
                                                            (line  8547)
* point sizes, fractional <1>:           Other Differences. (line 12546)
* polygon, drawing (\D'p ...'):          Drawing Requests.  (line 10230)
* polygon, solid, drawing (\D'P ...'):   Drawing Requests.  (line 10239)
* position of lowest text line (.h):     Diversions.        (line 10954)
* position, absolute (sic) operator (|): Numeric Expressions.
                                                            (line  4439)
* position, drawing:                     Page Geometry.     (line  4149)
* position, horizontal input line, saving (\k): Page Motions.
                                                            (line 10038)
* position, horizontal, in input line, register (hp): Page Motions.
                                                            (line 10044)
* position, horizontal, in output line, register (.k): Page Motions.
                                                            (line 10047)
* position, vertical, in diversion, register (.d): Diversions.
                                                            (line 10947)
* positions, font:                       Font Positions.    (line  7477)
* post-vertical line spacing:            Changing the Vertical Spacing.
                                                            (line  8522)
* post-vertical line spacing register (.pvs): Changing the Vertical Spacing.
                                                            (line  8534)
* post-vertical line spacing, changing (pvs): Changing the Vertical Spacing.
                                                            (line  8534)
* postprocessor access:                  Postprocessor Access.
                                                            (line 11677)
* postprocessors:                        Output Device Intro.
                                                            (line   369)
* PostScript fonts:                      Font Families.     (line  7370)
* PostScript, bounding box:              Miscellaneous.     (line 11912)
* PostScript, PNG image generation:      DESC File Format.  (line 13419)
* prefix, for commands:                  Environment.       (line   839)
* preprocessors:                         Preprocessor Intro.
                                                            (line   337)
* previous font (ft, \f[], \fP):         Changing Fonts.    (line  7291)
* previous line length (.n):             Environments.      (line 11350)
* print current page register (.P):      Groff Options.     (line   674)
* printing backslash (\\, \e, \E, \[rs]): Other Differences.
                                                            (line 12596)
* printing line numbers (nm):            Miscellaneous.     (line 11740)
* printing to stderr (tm, tm1, tmc):     Debugging.         (line 12073)
* printing, zero-width (\z, \Z):         Page Motions.      (line 10055)
* printing, zero-width (\z, \Z) <1>:     Page Motions.      (line 10059)
* process ID of GNU troff register ($$): Built-in Registers.
                                                            (line  5552)
* processing next file (nx):             I/O.               (line 11501)
* productive input line:                 Manipulating Filling and Adjustment.
                                                            (line  5749)
* properties of characters (cflags):     Using Symbols.     (line  7808)
* properties of glyphs (cflags):         Using Symbols.     (line  7808)
* ps request, and constant glyph space mode: Artificial Fonts.
                                                            (line  8200)
* ps request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12546)
* ps request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* ps request, with fractional type sizes: Using Fractional Type Sizes.
                                                            (line  8554)
* pso request, and safer mode:           Groff Options.     (line   711)
* pvs request, using + and - with:       Numeric Expressions.
                                                            (line  4431)
* quanta, motion:                        Motion Quanta.     (line  4260)
* quantum, horizontal motion:            DESC File Format.  (line 13415)
* quantum, vertical motion:              DESC File Format.  (line 13523)
* quotes, major:                         Displays.          (line  1262)
* radicalex glyph, and cflags:           Using Symbols.     (line  7844)
* ragged-left text:                      Manipulating Filling and Adjustment.
                                                            (line  5672)
* ragged-right text:                     Manipulating Filling and Adjustment.
                                                            (line  5668)
* rc request, and glyph definitions:     Using Symbols.     (line  7910)
* read-only register, changing format:   Assigning Register Formats.
                                                            (line  5431)
* reading from standard input (rd):      I/O.               (line 11506)
* recursive macros:                      while.             (line  9313)
* refer, and macro names starting with [ or ]: Identifiers. (line  4587)
* reference, gtroff:                     GNU troff Reference.
                                                            (line  3515)
* references [ms]:                       ms Insertions.     (line  2811)
* register format, in expressions:       Assigning Register Formats.
                                                            (line  5443)
* register, creating alias for (aln):    Setting Registers. (line  5282)
* register, format (\g):                 Assigning Register Formats.
                                                            (line  5438)
* register, removing (rr):               Setting Registers. (line  5272)
* register, removing alias for (aln):    Setting Registers. (line  5288)
* register, renaming (rnn):              Setting Registers. (line  5278)
* registers:                             Registers.         (line  5170)
* registers, built-in:                   Built-in Registers.
                                                            (line  5467)
* registers, dumping (pnr):              Debugging.         (line 12117)
* registers, interpolating (\n):         Interpolating Registers.
                                                            (line  5294)
* registers, number of, register (.R):   Built-in Registers.
                                                            (line  5488)
* registers, setting (nr, \R):           Setting Registers. (line  5179)
* removing a register (rr):              Setting Registers. (line  5272)
* removing alias for register (aln):     Setting Registers. (line  5288)
* removing alias, for diversion (rm):    Strings.           (line  8959)
* removing alias, for macro (rm):        Strings.           (line  8959)
* removing alias, for string (rm):       Strings.           (line  8959)
* removing diversion (rm):               Strings.           (line  8919)
* removing glyph definition (rchar, rfschar): Using Symbols.
                                                            (line  7967)
* removing macro (rm):                   Strings.           (line  8919)
* removing request (rm):                 Strings.           (line  8919)
* removing string (rm):                  Strings.           (line  8919)
* renaming a register (rnn):             Setting Registers. (line  5278)
* renaming diversion (rn):               Strings.           (line  8916)
* renaming macro (rn):                   Strings.           (line  8916)
* renaming request (rn):                 Strings.           (line  8916)
* renaming string (rn):                  Strings.           (line  8916)
* request:                               Requests and Macros.
                                                            (line  3780)
* request <1>:                           Formatter Instructions.
                                                            (line  4672)
* request arguments:                     Invoking Requests. (line  4749)
* request arguments, and compatibility mode: Gtroff Internals.
                                                            (line 12012)
* request arguments, and tabs:           Invoking Requests. (line  4749)
* request, removing (rm):                Strings.           (line  8919)
* request, renaming (rn):                Strings.           (line  8916)
* request, undefined:                    Comments.          (line  5110)
* requests for drawing:                  Drawing Requests.  (line 10082)
* requests for input and output:         I/O.               (line 11416)
* requests, intercepting:                Control Characters.
                                                            (line  4727)
* requests, invoking:                    Invoking Requests. (line  4741)
* requests, modifying:                   Control Characters.
                                                            (line  4727)
* resolution, device:                    Page Geometry.     (line  4137)
* resolution, device <1>:                DESC File Format.  (line 13476)
* resolution, device, obtaining in the formatter: Measurements.
                                                            (line  4208)
* resolution, horizontal:                DESC File Format.  (line 13415)
* resolution, horizontal, register (.H): Motion Quanta.     (line  4267)
* resolution, vertical:                  DESC File Format.  (line 13523)
* returning to marked vertical page location (rt): Page Motions.
                                                            (line  9822)
* revision number register (.Y):         Built-in Registers.
                                                            (line  5548)
* right margin:                          Line Layout.       (line  6852)
* right-justifying (rj):                 Manipulating Filling and Adjustment.
                                                            (line  5822)
* rivers:                                Other Differences. (line 12514)
* rj request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* rn glyph, and cflags:                  Using Symbols.     (line  7844)
* roman glyph, correction after italic glyph (\/): Ligatures and Kerning.
                                                            (line  8286)
* roman glyph, correction before italic glyph (\,): Ligatures and Kerning.
                                                            (line  8296)
* Roman numerals:                        Assigning Register Formats.
                                                            (line  5392)
* Roman numerals, extrema (maximum and minimum): Assigning Register Formats.
                                                            (line  5424)
* rq glyph, at end of sentence:          Sentences.         (line  3630)
* rq glyph, at end of sentence <1>:      Using Symbols.     (line  7854)
* rt request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* ru glyph, and cflags:                  Using Symbols.     (line  7844)
* running system commands:               I/O.               (line 11581)
* s scaling unit:                        Using Fractional Type Sizes.
                                                            (line  8554)
* safer mode:                            Groff Options.     (line   711)
* safer mode <1>:                        Macro Directories. (line   917)
* safer mode <2>:                        Built-in Registers.
                                                            (line  5492)
* safer mode <3>:                        I/O.               (line 11451)
* safer mode <4>:                        I/O.               (line 11564)
* safer mode <5>:                        I/O.               (line 11586)
* safer mode <6>:                        I/O.               (line 11625)
* safer mode <7>:                        Safer Mode.        (line 12357)
* saving horizontal input line position (\k): Page Motions. (line 10038)
* scaling indicator:                     Measurements.      (line  4200)
* scaling operator:                      Numeric Expressions.
                                                            (line  4358)
* scaling unit c:                        Measurements.      (line  4220)
* scaling unit f:                        Colors.            (line  8662)
* scaling unit i:                        Measurements.      (line  4217)
* scaling unit m:                        Measurements.      (line  4245)
* scaling unit M:                        Measurements.      (line  4255)
* scaling unit n:                        Measurements.      (line  4249)
* scaling unit p:                        Measurements.      (line  4223)
* scaling unit P:                        Measurements.      (line  4227)
* scaling unit s:                        Using Fractional Type Sizes.
                                                            (line  8554)
* scaling unit u:                        Measurements.      (line  4213)
* scaling unit v:                        Measurements.      (line  4252)
* scaling unit z:                        Using Fractional Type Sizes.
                                                            (line  8554)
* searching fonts:                       Font Directories.  (line   937)
* searching macros:                      Macro Directories. (line   902)
* seconds, current time (seconds):       Built-in Registers.
                                                            (line  5496)
* sentence space:                        Sentences.         (line  3585)
* sentence space size register (.sss):   Manipulating Filling and Adjustment.
                                                            (line  5832)
* sentences:                             Sentences.         (line  3573)
* sequence, escape:                      Formatter Instructions.
                                                            (line  4678)
* setting diversion trap (dt):           Diversion Traps.   (line 10611)
* setting end-of-input trap (em):        End-of-input Traps.
                                                            (line 10751)
* setting input line number (lf):        Debugging.         (line 12059)
* setting input line trap (it, itc):     Input Line Traps.  (line 10624)
* setting registers (nr, \R):            Setting Registers. (line  5179)
* shading filled objects (\D'f ...'):    Drawing Requests.  (line 10213)
* shc request, and translations:         Character Translations.
                                                            (line  6703)
* site-specific directory:               Macro Directories. (line   922)
* site-specific directory <1>:           Font Directories.  (line   960)
* size of sentence space register (.sss): Manipulating Filling and Adjustment.
                                                            (line  5832)
* size of word space register (.ss):     Manipulating Filling and Adjustment.
                                                            (line  5832)
* size, optical, of a font:              Changing Fonts.    (line  7338)
* size, paper:                           Paper Format.      (line   973)
* size, size:                            Manipulating Type Size and Vertical Spacing.
                                                            (line  8394)
* sizes, fractional:                     Other Differences. (line 12546)
* sizes, fractional type:                Using Fractional Type Sizes.
                                                            (line  8547)
* skew, of last glyph (.csk):            Environments.      (line 11335)
* slant, font, changing (\S):            Artificial Fonts.  (line  8120)
* soft hyphen character, setting (shc):  Manipulating Hyphenation.
                                                            (line  5977)
* soft hyphen glyph (hy):                Manipulating Hyphenation.
                                                            (line  5977)
* solid circle, drawing (\D'C ...'):     Drawing Requests.  (line 10186)
* solid ellipse, drawing (\D'E ...'):    Drawing Requests.  (line 10196)
* solid polygon, drawing (\D'P ...'):    Drawing Requests.  (line 10239)
* SOURCE_DATE_EPOCH, environment variable: Environment.     (line   886)
* sp request, and no-space mode:         Manipulating Spacing.
                                                            (line  6407)
* sp request, and traps:                 Manipulating Spacing.
                                                            (line  6318)
* sp request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* space between sentences:               Sentences.         (line  3585)
* space between sentences register (.sss): Manipulating Filling and Adjustment.
                                                            (line  5832)
* space between words register (.ss):    Manipulating Filling and Adjustment.
                                                            (line  5832)
* space character, as delimiter:         Delimiters.        (line  5054)
* space characters, in expressions:      Numeric Expressions.
                                                            (line  4506)
* space, between sentences:              Manipulating Filling and Adjustment.
                                                            (line  5843)
* space, between words:                  Manipulating Filling and Adjustment.
                                                            (line  5839)
* space, discardable, horizontal:        Manipulating Filling and Adjustment.
                                                            (line  5863)
* space, discarded, in traps:            Manipulating Spacing.
                                                            (line  6318)
* space, hair (\^):                      Page Motions.      (line  9962)
* space, horizontal (\h):                Page Motions.      (line  9936)
* space, horizontal, unformatting:       Punning Names.     (line 11193)
* space, thin (\|):                      Page Motions.      (line  9957)
* space, unbreakable (\~):               Manipulating Filling and Adjustment.
                                                            (line  5623)
* space, unbreakable and unadjustable (\SP): Page Motions.  (line  9952)
* space, vertical, unit (v):             Measurements.      (line  4252)
* space, width of a digit (numeral) (\0): Page Motions.     (line  9968)
* spaces with ds:                        Strings.           (line  8796)
* spaces, in a macro argument:           Calling Macros.    (line  4816)
* spaces, leading and trailing:          Breaking.          (line  3724)
* spacing:                               Basics.            (line  1136)
* spacing, manipulating:                 Manipulating Spacing.
                                                            (line  6294)
* spacing, vertical:                     Page Geometry.     (line  4170)
* spacing, vertical <1>:                 Manipulating Type Size and Vertical Spacing.
                                                            (line  8394)
* special characters:                    Sentences.         (line  3630)
* special characters <1>:                Character Translations.
                                                            (line  6689)
* special characters [ms]:               ms Legacy Features.
                                                            (line  3371)
* special fonts:                         Using Symbols.     (line  7574)
* special fonts <1>:                     Special Fonts.     (line  8051)
* special fonts <2>:                     Font Description File Format.
                                                            (line 13588)
* special fonts, emboldening:            Artificial Fonts.  (line  8189)
* special request, and font translations: Changing Fonts.   (line  7323)
* special request, and glyph search order: Using Symbols.   (line  7574)
* spline, drawing (\D'~ ...'):           Drawing Requests.  (line 10208)
* springing a trap:                      Traps.             (line 10385)
* sqrtex glyph, and cflags:              Using Symbols.     (line  7844)
* ss request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12563)
* stack:                                 Environments.      (line 11220)
* stacking glyphs (\b):                  Drawing Requests.  (line 10304)
* standard input, reading from (rd):     I/O.               (line 11506)
* stderr, printing to (tm, tm1, tmc):    Debugging.         (line 12073)
* stops, tab:                            Tabs and Leaders.  (line  3752)
* string arguments:                      Strings.           (line  8754)
* string comparison:                     Operators in Conditionals.
                                                            (line  9060)
* string expansion (\*):                 Strings.           (line  8754)
* string interpolation (\*):             Strings.           (line  8754)
* string, appending (as):                Strings.           (line  8841)
* string, creating alias for (als):      Strings.           (line  8924)
* string, length of (length):            Strings.           (line  8861)
* string, removing (rm):                 Strings.           (line  8919)
* string, removing alias for (rm):       Strings.           (line  8959)
* string, renaming (rn):                 Strings.           (line  8916)
* strings:                               Strings.           (line  8732)
* strings [ms]:                          ms Legacy Features.
                                                            (line  3371)
* strings, multi-line:                   Strings.           (line  8805)
* strings, shared name space with macros and diversions: Identifiers.
                                                            (line  4632)
* stripping final newline in diversions: Punning Names.     (line 11193)
* stroke color:                          Colors.            (line  8628)
* stroke color name register (.m):       Colors.            (line  8694)
* structuring source code of documents or macro packages: Invoking Requests.
                                                            (line  4763)
* sty request, and changing fonts:       Changing Fonts.    (line  7279)
* sty request, and font positions:       Font Positions.    (line  7545)
* sty request, and font translations:    Changing Fonts.    (line  7323)
* styles, font:                          Font Families.     (line  7365)
* substring (substring):                 Strings.           (line  8879)
* subtraction:                           Numeric Expressions.
                                                            (line  4327)
* suppressing output (\O):               Suppressing Output.
                                                            (line 11357)
* suppression nesting level register:    Suppressing Output.
                                                            (line 11410)
* sv request, and no-space mode:         Page Control.      (line  7217)
* switching environments (ev):           Environments.      (line 11256)
* sy request, and safer mode:            Groff Options.     (line   711)
* sy request, disabled by default:       Safer Mode.        (line 12357)
* symbol:                                Using Symbols.     (line  7574)
* symbol table, dumping (pm):            Debugging.         (line 12111)
* symbol, defining (char):               Using Symbols.     (line  7910)
* symbols, using:                        Using Symbols.     (line  7566)
* system commands, running:              I/O.               (line 11581)
* system() return value register (systat): I/O.             (line 11614)
* tab character:                         Tabs and Leaders.  (line  3752)
* tab character encoding:                Tabs and Fields.   (line  6423)
* tab character, and translations:       Character Translations.
                                                            (line  6699)
* tab character, as delimiter:           Delimiters.        (line  5054)
* tab character, non-interpreted (\t):   Tabs and Fields.   (line  6427)
* tab repetition character (tc):         Tabs and Fields.   (line  6544)
* tab stop settings register (.tabs):    Tabs and Fields.   (line  6535)
* tab stops:                             Tabs and Leaders.  (line  3752)
* tab stops, default:                    Tabs and Fields.   (line  6435)
* tab, line-tabs mode:                   Tabs and Fields.   (line  6557)
* table of contents:                     Table of Contents. (line  1299)
* table of contents <1>:                 Leaders.           (line  6616)
* table of contents, creating [ms]:      ms TOC.            (line  3106)
* table, multi-page, example [ms]:       ms Insertions.     (line  2853)
* tables [ms]:                           ms Insertions.     (line  2811)
* tabs, and fields:                      Tabs and Fields.   (line  6423)
* tabs, and macro arguments:             Invoking Requests. (line  4749)
* tabs, and request arguments:           Invoking Requests. (line  4749)
* tabs, before comments:                 Comments.          (line  5105)
* terminal, conditional output for:      Operators in Conditionals.
                                                            (line  9005)
* text baseline:                         Page Geometry.     (line  4152)
* text baseline <1>:                     Manipulating Type Size and Vertical Spacing.
                                                            (line  8394)
* text line:                             Requests and Macros.
                                                            (line  3793)
* text line, position of lowest (.h):    Diversions.        (line 10954)
* text, GNU troff processing:            Text.              (line  3522)
* text, justifying:                      Manipulating Filling and Adjustment.
                                                            (line  5577)
* text, justifying (rj):                 Manipulating Filling and Adjustment.
                                                            (line  5822)
* thickness of lines (\D't ...'):        Drawing Requests.  (line 10278)
* thin space (\|):                       Page Motions.      (line  9957)
* three-part title (tl):                 Page Layout.       (line  7089)
* ti request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5577)
* ti request, using + and - with:        Numeric Expressions.
                                                            (line  4431)
* time, current, hours (hours):          Built-in Registers.
                                                            (line  5502)
* time, current, minutes (minutes):      Built-in Registers.
                                                            (line  5499)
* time, current, seconds (seconds):      Built-in Registers.
                                                            (line  5496)
* time, formatting:                      I/O.               (line 11604)
* title line (tl):                       Page Layout.       (line  7089)
* title line length register (.lt):      Page Layout.       (line  7117)
* title line, length (lt):               Page Layout.       (line  7117)
* titles:                                Page Layout.       (line  7085)
* tkf request, and font styles:          Font Families.     (line  7419)
* tkf request, and font translations:    Changing Fonts.    (line  7323)
* tkf request, with fractional type sizes: Using Fractional Type Sizes.
                                                            (line  8554)
* tl request, and mc:                    Miscellaneous.     (line 11876)
* tmac, directory:                       Macro Directories. (line   907)
* tmac, path:                            Macro Directories. (line   907)
* TMPDIR, environment variable:          Environment.       (line   873)
* token, input:                          Gtroff Internals.  (line 11928)
* top margin:                            Page Layout.       (line  7074)
* top-level diversion:                   Diversions.        (line 10874)
* top-level diversion, and bp:           Page Control.      (line  7170)
* top-level diversion, and \!:           Diversions.        (line 11045)
* top-level diversion, and \?:           Diversions.        (line 11050)
* tr request, and glyph definitions:     Using Symbols.     (line  7910)
* tr request, and soft hyphen character: Manipulating Hyphenation.
                                                            (line  5977)
* tr request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12568)
* track kerning:                         Ligatures and Kerning.
                                                            (line  8259)
* track kerning, activating (tkf):       Ligatures and Kerning.
                                                            (line  8266)
* trailing double quotes in strings:     Strings.           (line  8796)
* trailing spaces in string definitions and appendments: Strings.
                                                            (line  8775)
* trailing spaces on text lines:         Breaking.          (line  3724)
* translations of characters:            Character Translations.
                                                            (line  6670)
* transparent characters:                Using Symbols.     (line  7854)
* transparent dummy character (\)):      Dummy Characters.  (line  8363)
* transparent output (cf, trf):          I/O.               (line 11468)
* transparent output (\!, \?):           Diversions.        (line 11013)
* transparent output, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12596)
* trap:                                  Deferring Output.  (line 10328)
* trap, changing location (ch):          Page Location Traps.
                                                            (line 10496)
* trap, distance to next vertical position, register (.t): Page Location Traps.
                                                            (line 10488)
* trap, diversion, setting (dt):         Diversion Traps.   (line 10611)
* trap, end-of-input, setting (em):      End-of-input Traps.
                                                            (line 10751)
* trap, input line, clearing (it, itc):  Input Line Traps.  (line 10624)
* trap, input line, setting (it, itc):   Input Line Traps.  (line 10624)
* trap, planting:                        Traps.             (line 10384)
* trap, springing:                       Traps.             (line 10385)
* traps:                                 Traps.             (line 10378)
* traps, and discarded space:            Manipulating Spacing.
                                                            (line  6318)
* traps, and diversions:                 Page Location Traps.
                                                            (line 10590)
* traps, blank line:                     Blank Line Traps.  (line 10722)
* traps, diversion:                      Diversion Traps.   (line 10610)
* traps, end-of-input:                   End-of-input Traps.
                                                            (line 10750)
* traps, input line:                     Input Line Traps.  (line 10622)
* traps, input line, and interrupted lines (itc): Input Line Traps.
                                                            (line 10648)
* traps, leading space:                  Leading Space Traps.
                                                            (line 10732)
* traps, page location:                  Page Location Traps.
                                                            (line 10409)
* traps, page location, dumping (ptr):   Debugging.         (line 12121)
* traps, page location, listing (ptr):   Debugging.         (line 12121)
* traps, sprung by bp request (.pe):     Page Location Traps.
                                                            (line 10568)
* traps, vertical position:              Vertical Position Traps.
                                                            (line 10390)
* trf request, and copy mode:            I/O.               (line 11468)
* trf request, and invalid characters:   I/O.               (line 11490)
* trf request, causing implicit break:   Manipulating Filling and Adjustment.
                                                            (line  5577)
* trin request, and asciify:             Diversions.        (line 11067)
* troff mode:                            troff and nroff Modes.
                                                            (line  6784)
* troff output:                          gtroff Output.     (line 12650)
* truncated vertical space register (.trunc): Page Location Traps.
                                                            (line 10556)
* truncating division:                   Numeric Expressions.
                                                            (line  4327)
* TTY, conditional output for:           Operators in Conditionals.
                                                            (line  9005)
* tutorial for macro users:              Tutorial for Macro Users.
                                                            (line  1047)
* type size:                             Manipulating Type Size and Vertical Spacing.
                                                            (line  8394)
* type size registers (.s, .ps):         Changing the Type Size.
                                                            (line  8429)
* type size registers, last-requested (.psr, .sr): Using Fractional Type Sizes.
                                                            (line  8588)
* type sizes, changing (ps, \s):         Changing the Type Size.
                                                            (line  8422)
* type sizes, fractional:                Using Fractional Type Sizes.
                                                            (line  8547)
* type sizes, fractional <1>:            Other Differences. (line 12546)
* TZ, environment variable:              Environment.       (line   893)
* u scaling unit:                        Measurements.      (line  4213)
* uf request, and font styles:           Font Families.     (line  7419)
* ul glyph, and cflags:                  Using Symbols.     (line  7844)
* ul request, and font translations:     Changing Fonts.    (line  7323)
* Ultrix-specific man macros:            Optional man extensions.
                                                            (line  1421)
* unadjustable and unbreakable space (\SP): Page Motions.   (line  9952)
* unary arithmetic operators:            Numeric Expressions.
                                                            (line  4333)
* unbreakable and unadjustable space (\SP): Page Motions.   (line  9952)
* unbreakable space (\~):                Manipulating Filling and Adjustment.
                                                            (line  5623)
* undefined identifiers:                 Identifiers.       (line  4620)
* undefined request:                     Comments.          (line  5110)
* underline font (uf):                   Artificial Fonts.  (line  8165)
* underlining (ul):                      Artificial Fonts.  (line  8139)
* underlining, continuous (cu):          Artificial Fonts.  (line  8161)
* underscore glyph (\[ru]):              Drawing Requests.  (line 10103)
* unformatting diversions (asciify):     Diversions.        (line 11067)
* unformatting horizontal space:         Punning Names.     (line 11193)
* Unicode:                               Identifiers.       (line  4545)
* Unicode <1>:                           Using Symbols.     (line  7770)
* unit, scaling, c:                      Measurements.      (line  4220)
* unit, scaling, f:                      Colors.            (line  8662)
* unit, scaling, i:                      Measurements.      (line  4217)
* unit, scaling, m:                      Measurements.      (line  4245)
* unit, scaling, M:                      Measurements.      (line  4255)
* unit, scaling, n:                      Measurements.      (line  4249)
* unit, scaling, p:                      Measurements.      (line  4223)
* unit, scaling, P:                      Measurements.      (line  4227)
* unit, scaling, s:                      Using Fractional Type Sizes.
                                                            (line  8554)
* unit, scaling, u:                      Measurements.      (line  4213)
* unit, scaling, v:                      Measurements.      (line  4252)
* unit, scaling, z:                      Using Fractional Type Sizes.
                                                            (line  8554)
* units of measurement:                  Measurements.      (line  4200)
* units, basic:                          Page Geometry.     (line  4137)
* units, basic, conversion to:           Measurements.      (line  4207)
* units, default:                        Default Units.     (line  4286)
* units, machine:                        Page Geometry.     (line  4137)
* unnamed fill colors (\D'F...'):        Drawing Requests.  (line 10288)
* unnamed glyphs:                        Using Symbols.     (line  7780)
* unnamed glyphs, accessing with \N:     Font Description File Format.
                                                            (line 13609)
* unsafe mode:                           Groff Options.     (line   787)
* unsafe mode <1>:                       Macro Directories. (line   917)
* unsafe mode <2>:                       Built-in Registers.
                                                            (line  5492)
* unsafe mode <3>:                       I/O.               (line 11451)
* unsafe mode <4>:                       I/O.               (line 11564)
* unsafe mode <5>:                       I/O.               (line 11586)
* unsafe mode <6>:                       I/O.               (line 11625)
* up-casing a string (stringup):         Strings.           (line  8897)
* uppercasing a string (stringup):       Strings.           (line  8897)
* upright glyph, correction after oblique glyph (\/): Ligatures and Kerning.
                                                            (line  8286)
* upright glyph, correction before oblique glyph (\,): Ligatures and Kerning.
                                                            (line  8296)
* URLs, breaking (\:):                   Manipulating Hyphenation.
                                                            (line  5953)
* user's macro tutorial:                 Tutorial for Macro Users.
                                                            (line  1047)
* user's tutorial for macros:            Tutorial for Macro Users.
                                                            (line  1047)
* using escape sequences:                Using Escape Sequences.
                                                            (line  4902)
* using symbols:                         Using Symbols.     (line  7566)
* UTF-8, output encoding:                Groff Options.     (line   755)
* v scaling unit:                        Measurements.      (line  4252)
* valid numeric expression:              Numeric Expressions.
                                                            (line  4490)
* value, incrementing without changing the register: Auto-increment.
                                                            (line  5362)
* variables in environment:              Environment.       (line   831)
* vee:                                   Page Geometry.     (line  4170)
* vee scaling unit (v):                  Measurements.      (line  4252)
* version number, major, register (.x):  Built-in Registers.
                                                            (line  5540)
* version number, minor, register (.y):  Built-in Registers.
                                                            (line  5544)
* vertical drawing position (nl):        Page Control.      (line  7224)
* vertical line drawing (\L):            Drawing Requests.  (line 10123)
* vertical line spacing register (.v):   Changing the Vertical Spacing.
                                                            (line  8493)
* vertical line spacing, changing (vs):  Changing the Vertical Spacing.
                                                            (line  8493)
* vertical line spacing, effective value: Changing the Vertical Spacing.
                                                            (line  8508)
* vertical motion (\v):                  Page Motions.      (line  9892)
* vertical motion quantum:               DESC File Format.  (line 13523)
* vertical page location, marking (mk):  Page Motions.      (line  9822)
* vertical page location, returning to marked (rt): Page Motions.
                                                            (line  9822)
* vertical position in diversion register (.d): Diversions. (line 10947)
* vertical position trap enable register (.vpt): Vertical Position Traps.
                                                            (line 10397)
* vertical position traps:               Vertical Position Traps.
                                                            (line 10390)
* vertical position traps, enabling (vpt): Vertical Position Traps.
                                                            (line 10397)
* vertical position, drawing (nl):       Page Control.      (line  7224)
* vertical resolution:                   DESC File Format.  (line 13523)
* vertical space unit (v):               Measurements.      (line  4252)
* vertical spacing:                      Page Geometry.     (line  4170)
* vertical spacing <1>:                  Manipulating Type Size and Vertical Spacing.
                                                            (line  8394)
* warning categories:                    Warnings.          (line 12209)
* warning level (warn):                  Debugging.         (line 12186)
* warnings:                              Debugging.         (line 12181)
* warnings <1>:                          Warnings.          (line 12200)
* what is groff?:                        What Is groff?.    (line   262)
* while:                                 while.             (line  9281)
* while request, and font translations:  Changing Fonts.    (line  7323)
* while request, and the ! operator:     Numeric Expressions.
                                                            (line  4333)
* while request, confusing with br:      while.             (line  9347)
* while request, operators to use with:  Operators in Conditionals.
                                                            (line  8979)
* width escape (\w):                     Page Motions.      (line  9981)
* width, of last glyph (.w):             Environments.      (line 11335)
* word space size register (.ss):        Manipulating Filling and Adjustment.
                                                            (line  5832)
* word, definition of:                   Filling.           (line  3547)
* write request, and copy mode:          I/O.               (line 11631)
* writec request, and copy mode:         I/O.               (line 11631)
* writem request, and copy mode:         I/O.               (line 11645)
* writing macros:                        Writing Macros.    (line  9364)
* writing to file (write, writec):       I/O.               (line 11631)
* year, current, register (year, yr):    Built-in Registers.
                                                            (line  5514)
* z scaling unit:                        Using Fractional Type Sizes.
                                                            (line  8554)
* zero-width printing (\z, \Z):          Page Motions.      (line 10055)
* zero-width printing (\z, \Z) <1>:      Page Motions.      (line 10059)
* zoom factor of a font (fzoom):         Changing Fonts.    (line  7338)

