nomore<: A System for Computing Preferred Answer Sets

The integration of preferences into Answer Set Programming (ASP) constitutes an important practical device for distinguishing certain preferred answer sets from non-preferred ones. Up to now, the preference semantics we are considering in this system description were incorporated into answer set solvers either by meta-interpretation [3] or by pre-compilation front-ends [2]; therefore, such kinds of preferences were never integrated into the core existing ASP solvers. Unlike this, the nomore system pursues an integrative approach to preference handling. Its theoretical background is described in [6]. The system itself is an early branch of the nomore++ ASP solver [8]. The approach relies on rule dependency graphs and computes answer sets by coloring this graph by following a certain strategy. It integrates preferences as an additional type of edges among nodes representing rules. The idea is to start from an uncolored graph and to employ specific operators that turn a partially colored graph gradually into a totally colored one that represents a preferred answer set. Apart from describing the nomore system, we focus in our experimental section on the question how an integrative approach compares to a compilationbased approach. To this end, we use the plp system [9] in connection with nomore as well as smodels [11] as its respective back-end ASP solvers. (Ab)using nomore as a standard ASP solver allows us to obtain comparable results, although its performance is much more inferior than that of smodels as well as the full-fledged nomore++ system [8].