Vladimir Lifschitz, Lappoon R. Tang and Hudson Turner
We extend the answer set semantics to a class of logic programs with nested expressions permitted in the bodies and heads of rules. These expressions are formed from literals using negation as failure, conjunction (,) and disjunction (;) that can be nested arbitrarily. Conditional expressions are introduced as abbreviations. The study of equivalent transformations of programs with nested expressions shows that any such program is equivalent to a set of disjunctive rules, possibly with negation as failure in the heads. The generalized answer set semantics is related to the Lloyd? Topor generalization of Clark's completion and to the logic of minimal belief and negation as failure.
Annals of Mathematics and Artificial Intelligence, Vol. 25 (1999), pp. 369-389.

Vladimir Lifschitz Faculty vl [at] cs utexas edu
Hudson Turner Ph.D. Alumni hudson [at] d umn edu