POPLOG

   A multi-language programming environment, which includes the
   languages Pop-11, ML, Common Lisp and Prolog.  It
   supports mixed-language programming and incremental
   compilation and includes a comprehensive X Window System
   interface.  It is built on top of a two-stack virtual
   machine, PVM.  POPLOG was developed at the University of
   Sussex, Brighton, UK.

   ["POPLOG's Two-Level Virtual Machine Support for Interactive
   Languages", R. Smith et al, in Research Directions in
   Cognitive Science, v.5 (1992)].