Now find a copy of Computers and Thought --- a practical introduction to AI by Sharples, Hogg, Hutchinson, Torrance and Young (MIT press, 1989). Read the production system part of Chapter 8 (on childrens' subtraction), and follow up the references to the work of Young and O'Shea, in particular their article in Cognitive Science for 1981.
Note their (generally favourable) comments on Brown and Van Lehn's work on repair theory. There is a paper on this in Cognitive Science for 1980. More recently, Van Lehn has written a book called Mindbugs, now available in the library. If you have time, read at least the introduction.