Syntax:    bf(%no-lines) 

Do not put bf(#line) preprocessor directives in the file containing the
parser's tt(parse) function. By default tt(#line) preprocessor directives
are inserted just before action blocks in the generated tt(parse.cc) file.

The tt(#line) directives allow compilers and debuggers to associate errors
with lines in your grammar specification file, rather than with the source
file containing the tt(parse) function itself.
