Syntax:    bf(%implementation-header) tt(filename) nl()
           tt(Filename) defines the name of the file to contain the
            implementation header. It defaults to the name of the generated
            parser class plus the suffix tt(.ih). nl()
           The implementation header should contain all directives and
            declarations em(only) used by the implementations of the parser's
            member functions. It is the only header file that is included by
            the source file containing tt(parse)'s implementation. User
            defined implementation of other class members may use the same
            convention, thus concentrating all directives and declarations
            that are required for the compilation of other source files
            belonging to the parser class in one header file.nl()
           This directive is overruled by the
            bf(--implementation-header) (bf(-i)) command-line option.
