## Abstract

We propose a modification $\call_1$ of the action description language ${\cal A}$. The language $\call_1$ allows representation of hypothetical situations and hypothetical occurrence of actions (as in $\cala$) 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 $\call_1$ we also present an architecture for intelligent agents capable of observing, planning and acting in a changing environment based on the entailment relation of $\call_1$ 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.