In this paper we give the notion of modularity of a theory and analyze some of its properties, especially for the case of action theories in reasoning about actions. We propose algorithms to check whether a given action theory is modular and that also make it modular, if needed. Completeness, correctness and termination results are demonstrated.

herzig,ivan¢ In this work we recast some design principles commonly used in software engineering and adapt them to the design and analysis of domain descriptions in reasoning about actions. We show how the informal requirements of cohesion and coupling can be turned into consistency tests of several different arrangements of modules. This gives us new criteria for domain description evaluation and clarifies the link between software and knowledge engineering in what concerns the meta-theory of actions. 1

In this work we address the problem of elaborating domain descriptions (alias action theories), in particular those that are expressed in dynamic logic. We define a general method based on contraction of formulas in a version of propositional dynamic logic with an incorporated solution to the frame problem. We present the semantics of our theory change and define syntactical operators for contracting a domain description. We establish

Abstract. In this paper we address the principle of modularity of ontologies in description logics. It turns out that with existing accounts of modularity of ontologies we do not completely avoid unforeseen interactions between module components, and modules designed in those ways may be as complex as whole theories. We here give a more finegrained paradigm for modularizing descriptions. We propose algorithms that check whether a given terminology is modular and that also help the designer making it modular, if needed. Completeness, correctness and termination results are demonstrated for a fragment of ALC. We also present the properties that ontologies that are modular in our sense satisfy w.r.t. reasoning services.

Consistency check has been the only criterion for theory evaluation in logic-based approaches to reasoning about actions. This work goes beyond that and contributes to the metatheory of actions by investigating what other properties a good domain description in reasoning about actions should have. We state some metatheoretical postulates concerning this sore spot. When all postulates are satisfied together we have a modular action theory. Besides being easier to understand and more elaboration tolerant in McCarthy's sense, modular theories have interesting properties. We point out the problems that arise when the postulates about modularity are violated and propose algorithmic checks that can help the designer of an action theory to overcome them.

Like any other logical theory, domain descriptions in reasoning about actions may evolve, and thus need revision methods to adequately accommodate new information about the behavior of actions. The present work is about changing action domain descriptions in propositional dynamic logic. Its contribution is threefold: first we revisit the semantics of action theory contraction that has been done in previous work, giving more robust operators that express minimal change based on a notion of distance between Kripke-models. Second we give algorithms for syntactical action theory contraction and establish their correctness w.r.t. our semantics. Finally we state postulates for action theory contraction and assess the behavior of our operators w.r.t. them. Moreover, we also address the revision counterpart of action theory change, showing that it benefits from our semantics for