Not sure what the MIT implementation is, but thats the one I used in uni. <a href="http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-001-structure-and-interpretation-of-computer-programs-spring-2005/video-lectures/1a-overview-and-introduction-to-lisp/" rel="nofollow">http://ocw.mit.edu/courses/electrical-engineering-and-comput...</a>