I don't see how you could make a "mock" programming language implementation. Either it's a complete implementation, in which case it's just as real as any other implementation, or it isn't, in which case it's an implementation of another language.<p>Incidentally, the reason we didn't make our implementation too Mzscheme-specific is that we wanted to make it portable across Scheme versions. In fact, it started out written on another version of Scheme, Scheme48.