Hypothetical Updates, Priority and Inconsistency in a Logic Programming Language

1 I n t r o d u c t i o n In [8] we have proposed a logic p rog ramming language which suppor ts hypothe t ical updates together with integrity constraints. The language, called CondLP (Conditional Logic Programming), is an extension of N_Prolog [10, 7]. As in N_Prolog and in other similar logic p rog ramming languages [16, 15], Horn clause logic is extended to allow embedded implications bo th in goals and in clause bodies. In this paper we present a language, CondLP + which extends CondLP in m a n y different ways. Operat ional ly, in N_Prolog an embedded implicat ion D =~ G, where G is a goal and D is a set of clauses, succeeds f rom a p rogram P if G succeeds f rom the enlarged p rogram P U D. Hence, D can be regarded as an hypothet ica l upda te to the current program, tha t only affects the proof of G. As a difference with N_Prolog, in bo th CondLP and CondLP + a p rogram m a y contain integri ty constraints, and inconsistencies m a y arise, when updates are performed. In bo th languages we have a restriction on the allowed updates: in CondLP D is a single a tom, while in CondLP + D is a set of a toms. This work has been partially supported by ESPRIT Basic Research Project 6471, Medlar If.