I would email Prof. Friedman, I've spoke with him in the past and he was very friendly and extremely eager to discuss his work (of course keep in mind he is very busy). As far as I could tell chatting with him the whole Kanren system is a continually evolving project to make a better and better logic programming system in scheme. He's an amazing hacker and extremely dedicated to pushing Scheme to it's absolute limits, I remember him showing me all the places where him and his co-author were able to reduce 6 loc to 4 here and 10 to 8 there etc. You could tell that he's continually reviewing code saying "I know this can be more abstract, more simple but how?" I think they've reduced the implementation used in the reasoned schemer from 2 to 1 1/2 pages at this point (edit: originally said 6 to 4 pages but reviewing some notes from the meeting, after seeing keithflower's reply, I've adjusted it accordingly ). If you can't get in touch with Friedman definitely try some of the other co-authors, Friedman gives them a ton of credit for making the system as awesome as it is today.