1. Towards a theory of declarative knowledge;Apt,1988
2. Saturation, nonmonotonic reasoning and the closed world assumption;Bossu;Artificial Intelligence,1985
3. Horn clause queries and generalizations;Chandra;J. Logic Program.,1985
4. Negation as failure;Clark,1978
5. Reasoning with incomplete information, investigations of non-monotonic reasoning;Etherington,1986