 
Summary: Reliable Semantics for Extended Logic Programs with
Rule Prioritization
Anastasia Analyti & Sakti Pramanik
Department of Computer Science, Michigan State University, E. Lansing, MI 48824
Email: analyti@cps.msu.edu, pramanik@cpswh.msu.edu
Abstract
We present a new semantics for extended logic programs with rule prioritization (EPP). The semantics,
called reliable semantics (RS), generalizes the wellfounded semantics which is defined for normal programs
[34]. It also generalizes the extended wellfounded semantics which is defined for noncontradictory extended
programs [24]. Because of the classical negation in the head of the rules, the wellfounded model of a
program can be contradictory. RS is contradictionfree and defined for all EPPs. To avoid contradictions,
only reliable rules and reliable closed world assumptions are used in the fixpoint computation of RS. A
closed world assumption (CWA) is considered reliable if it is not "suspect" for any constraint violation. A
rule r is considered reliable if it is not "suspect" for any constraint violation caused by rules with priority
no lower than r. We define the stable rmodels of an EPP and we show that the RS of a program coincides
with its least stable rmodel. The RS of an EPP, P, represents the skeptical "meaning" of P whereas stable
rmodels of P represent the possible "meanings" of P.
Keywords: extended logic programs, rule prioritization, constraint violation, semantics.
1 Introduction
Extended programs are normal programs extended with classical negation. Extended programs pro
