I've been a student of Lisp(s) for roughly five years now. Unfortunately, sometimes, pretty simple bugs show up and bite me. I'm a bit embarrassed to say this but... I haven't written anything about MPS for a while because I've had a problem with the proof-of-concept code. Or, so I thought. It turned out that the agenda function used mapcan instead of mapcar. Funny (well...) thing is that since I thought it used mapcar it took me quite a while to find it.