Interested in multi-language development? Study Poplog, "a free, open source, multi-language software development environment providing incremental compilers for a number of interactive programming languages". Poplog comes with four languages: Common Lisp, ML, Pop-11 and Prolog. These languages can use libraries written in other languages and the users can freely switch between them in the middle of a Poplog interactive session. Even better, users can embed languages! Here is documentation on Prolog and Pop-11; ML and Pop-11; Lisp and Pop-11; Proplog and Lisp.