The Free On-line Dictionary of Computing (30 December 2018):

Decus cpp

   An almost-ANSI C preprocessor by Martin Minow.  It is
   shipped with X11R5 (contrib/util/cpp) because some systems
   don't have a working cpp.

   It runs on VMS (Vax C, Decus C), RSX-11M, RSTS/E,
   P/OS, RT11, A/UX and Apollo Domain/IX 9.6 and is
   highly portable.

   (1994-11-03)