by Maurizio Lenzerini
We study the problem of changing an ontology expressed as a Description Logic knowledge base. Such a knowledge base is constituted by two components, called TBox and ABox. The former expresses general knowledge about the concepts and their relationships, whereas the latter describes the state of affairs regarding the instances of concepts. We investigate the case where the change affects only the instance level of the ontology, i.e., the ABox. Building on classical approaches on knowledge base update and erasure, we first provide a general semantics for instance level update and erasure in Description Logics. We then focus on DL-Lite, a specific Description Logic where the basic reasoning tasks are computationally tractable. We show that DL-Lite, like many other Description Logics, is not closed with respect to instance level update, in the sense that the set of models corresponding to an update applied to a knowledge base expressed in DL-Lite may not be expressible by ABoxes in DL-Lite. The same holds for erasure. Finally, we present an approach for overcoming the expressibility problem, together with efficient algorithms for computing the result of an update and of an erasure within this approach.