A Note on Reasoning on DL-Lite_R with Defeasibility

Representation of defeasible information is of interest in description logics, as it is related to the need of accommodating exceptional instances in knowledge bases. In this direction, in our previous works we presented a datalog translation for reasoning on (contextualized) OWL RL knowledge bases with a notion of justified exceptions on defeasible axioms. While it covers a relevant fragment of OWL, the resulting reasoning process needs a complex encoding in order to capture reasoning on negative information. In this paper, we consider the case of knowledge bases in $ extitDL-Lite_cal R$, i.e. the language underlying OWL QL. We provide a definition for $ extitDL-Lite_cal R$ knowledge bases with defeasible axioms and study their properties. The limited form of $ extitDL-Lite_cal R$ axioms allows us to formulate a simpler encoding into datalog (under answer set semantics) with direct rules for reasoning on negative information. The resulting materialization method gives rise to a complete reasoning procedure for instance checking in $ extitDL-Lite_cal R$ with defeasible axioms.