Abstract

. A method for extracting positive information from negative goals is proposed. The method makes use of knowledge about the existence of solutions of predicates and about the types of variables. It strictly generalises earlier work in this area. It can be used as a construction rule or as a simplification rule. As a construction rule it does not involve an SLD-derivation of the negated sub-goal and so is applicable during compilation. As a simplification rule it prunes unsatisfiable goals without doing an explicit satisfiability test. Keywords: constructive negation, existence properties, type system 1 Introduction The "negation by failure" rule, NAF for short, may lead to floundering when negative goals contain variables. The reason is that NAF doesn't allow negative goals to bind variables. To overcome this limitation, Chan introduces the "constructive negation" rule which allows non-ground negative goals to bind variables in the same way as positive ones [2, 3]. The basic idea i...

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