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* doc/common.x, doc/bison.x: New.
* doc/Makefile.am (bison.1, .x.1): New.
The code is taken from autoconf-2.61/man/Makefile.am.
* configure.ac: Look for help2man.

doc/bison.x

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+[name]
+bison \- GNU Project parser generator (yacc replacement)
+
+[description]
+.I Bison
+is a parser generator in the style of
+.IR yacc (1).
+It should be upwardly compatible with input files designed
+for
+.IR yacc .
+.PP
+Input files should follow the
+.I yacc
+convention of ending in
+.BR .y .
+Unlike
+.IR yacc ,
+the generated files do not have fixed names, but instead use the prefix
+of the input file.
+Moreover, if you need to put
+.I C++
+code in the input file, you can end his name by a C++-like extension
+(.ypp or .y++), then bison will follow your extension to name the
+output file (.cpp or .c++).
+For instance, a grammar description file named
+.B parse.yxx
+would produce the generated parser in a file named
+.BR parse.tab.cxx ,
+instead of
+.IR yacc 's
+.B y.tab.c
+or old
+.I Bison
+version's
+.BR parse.tab.c .
+.PP
+This description of the options that can be given to
+.I bison
+is adapted from the node
+.B Invocation
+in the
+.B bison.texinfo
+manual, which should be taken as authoritative.
+.PP
+.I Bison
+supports both traditional single-letter options and mnemonic long
+option names.  Long option names are indicated with
+.B \-\-
+instead of
+.BR \- .
+Abbreviations for option names are allowed as long as they
+are unique.  When a long option takes an argument, like
+.BR \-\-file-prefix ,
+connect the option name and the argument with
+.BR = .