    Syntax: bf(%baseclass-header) tt(filename) nl()
           tt(Filename) defines the name of the file to contain the parser's
            base class. This class defines, e.g., the parser's symbolic
            tokens. Defaults to the name of the parser class plus the suffix
            tt(base.h). It is always generated, unless (re)writing is
            suppressed by the tt(--no-baseclass-header) and
            tt(--dont-rewrite-baseclass-header) options. This directive is
            overruled by the bf(--baseclass-header) (bf(-b)) command-line
            option.

It is an error if this directive is used and an already existing parser class
header file does not contain tt(#include "filename").
