One important facet of common-sense reasoning is the ability to draw default conclusions about the state of the world, so that one can, for example, assume that a given bird flies in the absence of information to the contrary. A deficiency in the circumscriptive approach to common-sense reasoning has been its difficulties in producing default that Tweety ≠ Blutto using ordinary circumscription, or conclude by default that a particular bird flies, if some birds are known not to fly. In this paper, we introduce a new form of circumscription, based on homomorphisms between models, that remedies these two problems and still retains the major desirable properties of traditional forms of circumscription.

Current data structures for online secondary storage depend on the ability to alter data once it is written. On an optical disk such modification is not possible, so new approaches are needed. In this paper, data structures which allow additions, logical deletions, and rapid access to data stored on optical disks are proposed and analyzed.