m4

   A macro processor for Unix and GCOS which is more
   flexible than cpp.  m4 copies its input to the output,
   expanding macros which can be either built-in or user-defined.
   m4 has built-in functions for including files, running Unix
   commands, doing integer arithmetic, manipulating text in
   various ways and recursing.  m4 can be used either as a
   front-end to a compiler or as a stand-alone tool.

   sendmail's configuration file (/etc/sendmail.cf) is writen
   in m4 macros.

   There is a <GNU m4 v1.1> by Francois
   Pinard  and a public domain version
   by Ozan Yigit  and Richard A. O'Keefe
    (FTP from any 386BSD, NetBSD or
   FreeBSD archive).  A Macintosh version is <here>.

   See also m3, m5.

   ["The M4 Macro Processor",
   Kernighan & Ritchie, Jul 1977].