Preferred Answer Sets for Extended Logic Programs

In this paper, we extend Gelfond and Lifschitz’s answer set semantics to prioritized extended logic programs. In such programs, an ordering on the program rules is used to express preferences. We show how this ordering can be used to define preferred answer sets and thus to increase the set of consequences of a program. We define a strong and a weak notion of preferred answer sets. The first takes preferences more seriously and satisfies reasonable principles for priority handling in rule based systems, while the second guarantees the existence of a preferred answer set for consistent programs. We show that strong preference on answer sets does not add on the complexity of the principal reasoning tasks, and weak preference leads only to a mild increase in complexity.