The Expressive Power of Stratified Programs

The study of negation in logic programming has been the topic of substantial research activity during the past several years, starting with the negation as failure semantics in Clark (1978), and Apt and van Emden (1982). More recently, a major direction of research has focused on the class of stratified logic programs, in which no predicate is defined recursively in terms of its own negation and which can be given natural semantics in terms of iterated fixpoints. Stratified logic programs were introduced and studied first by Chandra and Hare1 (1985), but soon attracted the interest of researchers from both database theory and artificial intelligence. Recent research work on stratified logic programs and their generalizations includes the papers by Apt, Blair, and Walker (1988), Van Gelder (1986) Lifschitz (1988), Przymusinski (1988), Apt and Pugin (1987) and others. At the same time, stratified logic programs became the choice for the treatment of negation in the NAIL ! system developed at Stanford University by Ullman and his co-workers (cf. Morris

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