Priorities in Default Logic revisited

Abstract

One of the drawbacks of Reiter's original presentation of Default Logic is the difficulty of expressing priorities among defaults. Recently, Brewka proposed an expansion of Default Logic which allows for explicit reasoning about priorities in Default Logic. In this paper we give an alternative, operational characterization of extensions; it is also technically simpler and therefore more easily applicable. Further we investigate some properties of the logic paying special attention to the existence of extensions. We present some sufficient conditions for a default theory with priorities to have at least one extension.