Reasoning with Justifiable Exceptions in Contextual Hierarchies

The problem of representing and reasoning with context dependent knowledge has been of certain interest since the beginning of AI. Among the available solutions, we consider the Contextualized Knowledge Repository (CKR) framework. In CKR applications it is often useful to reason over a hierarchical organization of contexts: however, the CKR model is not able to represent exception handling in the inheritance of knowledge across contexts. In this paper we develop a proposal, based on a recent principle for exception handling for inheritance in description logics, that allows CKRs with context dependent defeasible axioms which can be overridden by more specific local knowledge. We provide an alternative semantics for a core (simple) version of CKR that copes with contextual defeasible axioms, and we define a datalog translation generating programs that are complete w.r.t. instance checking under the proposed semantics in the case of ranked contextual hierarchies.