Revising default theories

Default logic is a prominent rigorous method of reasoning with incomplete information based on assumptions. It is a static reasoning approach, in the sense that it doesn't reason about changes and their consequences. On the other hand, its nonmonotonic behaviour appears when a change to a default theory is made. This paper studies the dynamic behaviour of default logic in the face of changes, a concept that we motivate by a reference to requirements engineering. The paper defines a contraction and a revision operator, and studies their properties. This work is part of an ongoing project whose aim is to build an integrated, domain-independent toolkit of logical methods for reasoning with changing and incomplete information. The techniques described in this paper will be implemented as part of the toolkit.