1. Charniak, E., Motivation analysis, abductive unification, and non-monotonic equality, Artificial Intelligence, to appear.
2. Negation as failure;Clark,1978
3. The mathematics of non-monotonic reasoning;Davis;Artificial Intelligence,1980
4. Formalizing non-monotonic reasoning systems;Etherington,1983
5. On inheritance hierarchies with exceptions;Etherington,1983