( Extensions

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 15.1 Extensions in GNU `m4'
 This version of `m4' contains a few facilities that do not exist in
 System V `m4'.  These extra facilities are all suppressed by using the
 `-G' command line option ( Invoking m4), unless overridden by
 other command line options.
    * In the `$'N notation for macro arguments, N can contain several
      digits, while the System V `m4' only accepts one digit.  This
      allows macros in GNU `m4' to take any number of arguments, and not
      only nine ( Arguments).
      This means that `define(`foo', `$11')' is ambiguous between
      implementations.  To portably choose between grabbing the first
      parameter and appending 1 to the expansion, or grabbing the
      eleventh parameter, you can do the following:
           define(`a1', `A1')
           dnl First argument, concatenated with 1
           define(`_1', `$1')define(`first1', `_1($@)1')
           dnl Eleventh argument, portable
           define(`_9', `$9')define(`eleventh', `_9(shift(shift($@)))')
           dnl Eleventh argument, GNU style
           define(`Eleventh', `$11')
           first1(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
           eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
           Eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
    * The `divert' ( Divert) macro can manage more than 9
      diversions.  GNU `m4' treats all positive numbers as valid
      diversions, rather than discarding diversions greater than 9.
    * Files included with `include' and `sinclude' are sought in a user
      specified search path, if they are not found in the working
      directory.  The search path is specified by the `-I' option and the
      `M4PATH' environment variable ( Search Path).
    * Arguments to `undivert' can be non-numeric, in which case the named
      file will be included uninterpreted in the output (
    * Formatted output is supported through the `format' builtin, which
      is modeled after the C library function `printf' ( Format).
    * Searches and text substitution through regular expressions are
      supported by the `regexp' ( Regexp) and `patsubst' (
      Patsubst) builtins.
    * The output of shell commands can be read into `m4' with `esyscmd'
      ( Esyscmd).
    * There is indirect access to any builtin macro with `builtin'
      ( Builtin).
    * Macros can be called indirectly through `indir' ( Indir).
    * The name of the program, the current input file, and the current
      input line number are accessible through the builtins
      `__program__', `__file__', and `__line__' ( Location).
    * The format of the output from `dumpdef' and macro tracing can be
      controlled with `debugmode' ( Debug Levels).
    * The destination of trace and debug output can be controlled with
      `debugfile' ( Debug Output).
    In addition to the above extensions, GNU `m4' implements the
 following command line options: `-F', `-G', `-I', `-L', `-R', `-V',
 `-W', `-d', `-i', `-l', `--debugfile' and `-t'.   Invoking m4,
 for a description of these options.
    Also, the debugging and tracing facilities in GNU `m4' are much more
 extensive than in most other versions of `m4'.
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