Reasoning on Open Domains

In this paper we modify the semantics of epistemic speci cations (and hence the answer set semantics of extended logic program and disjunctive databases) to allow for reasoning in the absence of domain{closure assumption. This modi cation increases the expressive power of the language and allows one to explicitly state the domain{closure and other assumptions about the domain of discourse in the language of epistemic speci cations. The power of the language is demonstrated by way of examples. In particular we show how open domain assumption can be used to formalize default reasoning in the presence of anonymous exceptions to defaults.