The AGM theory of belief contraction is extended tomultiple contraction, i.e. to contraction by a set of sentences rather than by a single sentence. There are two major variants: Inpackage contraction all the sentences must be removed from the belief set, whereas inchoice contraction it is sufficient that at least one of them is removed. Constructions of both types of multiple contraction are offered and axiomatically characterized. Neither package nor choice contraction can in general be reduced to contractions by single (...) sentences; in the finite case choice contraction allows for reduction. (shrink)

Investigates the resolution of contradictions and ambiguous derogations in a code, by means of the imposition of partial orderings. Although formulated as a study in the logic of norms, it provided the initial ideas for work on the logic of theory (or belief) change, developed by the authors in a series of papers by the authors and Peter Gardenfors beginning in 1985.