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

IMProved Mercury autocode

    (IMP) A version of Autocode used to program the
   Edinburgh Multi Access System (EMAS), one of the first
   operating systems written in a high-level language,
   apparently predating Unix.

   Luis Damas' Prolog interpreter in IMP for EMAS led to
   C-Prolog.

   [Papers in J. British Computer Society].

   (1996-04-07)