Theory and Practice of Logic Programming

Regular Papers

Weight constraints as nested expressions

a1 Department of Computer Sciences, University of Texas at Austin, Austin, TX 78712, USA (e-mail:,

Article author query
ferraris p   [Google Scholar] 
lifschitz v   [Google Scholar] 


We compare two recent extensions of the answer set (stable model) semantics of logic programs. One of them, due to Lifschitz, Tang and Turner, allows the bodies and heads of rules to contain nested expressions. The other, due to Niemelä and Simons, uses weight constraints. We show that there is a simple, modular translation from the language of weight constraints into the language of nested expressions that preserves the program's answer sets. Nested expressions can be eliminated from the result of this translation in favor of additional atoms. The translation makes it possible to compute answer sets for some programs with weight constraints using satisfiability solvers, and to prove the strong equivalence of programs with weight constraints using the logic of here-and-there.

Key Words: answer sets; cardinality constraints; SMODELS; stable models; weight constraints.