Source : Free On-Line Dictionary of Computing

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)