Reasoning with Prioritized Defaults

The purpose of this paper is to investigate the methodology of reasoning with prioritized defaults in the language of logic programs under the answer set semantics. We present a domain independent system of axioms, written as an extended logic program, which deenes reasoning with prioritized defaults. These axioms are used in conjunction with a description of a particular domain encoded in a simple language allowing representation of defaults and their priorities. Such domain descriptions are of course domain dependent and should be speciied by the users. We give suucient conditions for consistency of domain descriptions and illustrate the use of our system by formalizing various examples from the literature. Unlike many other approaches to formalizing reasoning with priorities ours does not require development of the new semantics of the language. Instead, the meaning of statements in the domain description is given by the system of (domain independent) axioms. We believe that in many cases this leads to simpler and more intuitive formalization of reasoning examples. We also present some discussion of diierences between various formalizations.