We propose a modification ℒ1 of the action description language script A sign. The language ℒ1 allows representation of hypothetical situations and hypothetical occurrence of actions (as in script A sign) as well as representation of actual occurrences of actions and observations of the truth values of fluents in actual situations. The corresponding entailment relation formalizes various types of common-sense reasoning about actions and their effects not modeled by previous approaches. As an application of ℒ1 we also present an architecture for intelligent agents capable of observing, planning and acting in a changing environment based on the entailment relation of ℒ1 and use logic programming approximation of this entailment to implement a planning module for this architecture. We prove the soundness of our implementation and give a sufficient condition for its completeness.
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