The event calculus and consistency maintenance

The relationship between nonmonotonic formalisms and the problem of temporal projection has long been recognized, though not well understood. The straightforward application of various nonmonotonic formalisms (circumscription, default logic, etc.) has been blocked by the multiple-extension problem [Hanks and McDermott 87], where a seemingly intuitive set of axioms gives rise to extensions (or minimal models, depending on the formalism) other than the one desired. However, recent work in the semantics of logic programs has gone a long way toward clarifying this situation. In particular, a class of logic programs (called the stratified logic programs) has been identified in which nonmonotonic reasoning can be carried out in a well-defined fashion [Przymusinski 88]. This paper presents a system, based on the Event Calculus [Kowalski and Sergot 86], for temporal reasoning in a deductive database context. The axioms used to perform temporal projection are constructed to satisfy the stratification property, so that nonmonotonic inference can be performed safely. The system employs a caching mechanism which stores conclusions drawn from the axioms along with justifications. The cache is managed using truth maintenance techniques which faithfully reflect the nonmonotonic semantics of the system.