Defeasibility in contextual reasoning with CKR

Recently, representation of context dependent knowledge in the Semantic
Web has been recognized as a relevant issue and a number of logic based
solutions have been proposed in this regard: among them, in our previous works
we presented the Contextualized Knowledge Repository (CKR) framework.
A CKR knowledge base has a two layered structure, modelled by a global context
and a set of local contexts: the global context not only contains the metaknowledge
defining the properties of local contexts, but also holds the global (context
independent) object knowledge that is shared by all of the local contexts. In many
practical cases, however, it is desirable to leave the possibility to “override” the
global object knowledge at the local level, by recognizing the axioms that can
allow exceptional instances in the local contexts. This clearly requires to add a
notion of non monotonicity across the global and the local parts of a CKR.
In this paper we present an extension to the semantics of CKR to introduce such
notion of defeasible axioms. By extending a previously proposed datalog translation,
we obtain a representation for CKR as a datalog program with negation
under answer set semantics. This representation can be exploited as the basis for
implementation of query answering for the proposed extension of CKR.