Keith Golden

Sound and efficient closed-world reasoning for planning

Abstract Closed-world inference—an essential component of many planning algorithms—is the process of determining that a logical sentence is false based on its absence from a knowledge base, or the inability to derive it. We describe a novel method for closed-world inference and update over the first-order theories of action used by planning algorithms such as nonlin, tweak and ucpop . We show the method to be sound and efficient, but incomplete. In our experiments, closed-world inference consistently averaged about 2 milliseconds while updates averaged approximately 1.2 milliseconds. Furthermore, we demonstrate that incompleteness is nonproblematic in practice, since our mechanism makes over 99% of the desired inferences. We incorporated our method into the xii planner, which supports our Internet Softbot (software robot). The technique cut the number of actions executed by the Softbot by a factor of one hundred, and resulted in a corresponding speedup to xii .

Tractable closed world reasoning with updates

Abstract Closed world reasoning is the process of inferring that a logical sentence is false based on its absence from a knowledge base, or the inability to derive it. Previous work on circumscription, autoepistemic logic, and database theory has explored logical axiomatizations of closed world reasoning, and investigated computational tractability for propositional theories. Work in planning has traditionally made the closed world assumption but has avoided closed world reasoning. We take a middle position, and describe a tractable method for closed world reasoning over the schematized theories of action used by planning algorithms such as NONLIN, TWEAK, and UCPOP. We show the method to be both sound and tractable, and incorporate it into the XII planner [Golden et al. , 1994]. Experiments utilizing our softbot (software robot) demonstrate that the method can substantially improve its performance by eliminating redundant information gathering.