POPLOGA 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)].