In local change, the operation ∘ is specific for the original belief set K. Formally it is a function that takes us from a descriptor Ψ to an element K∘ Ψ of the outcome set X (the set of belief sets that are potential outcomes of belief change). It only represents changes that have K as their starting-point. In this chapter the framework of descriptor revision is widened to global (iterated) belief change. This means that the operation ∘ can be applied to any potential belief set. Formally, it is a function that takes us from a pair consisting of a belief set K and a descriptor Ψ to a new belief set K∘ Ψ. This makes it possible to cover successive changes, such as K∘ Bp∘ ¬ Bp. Several constructions of global descriptor revision are presented and axiomatically characterized. The most orderly of these constructions is based on pseudodistances (distance measures that allow the distance from X to Y to differ from the distance from Y to X). For any elements X and Y of the outcome set, i.e. the set of belief sets that are eligible as outcomes, there is a number δ(X, Y) denoting how far away Y is from X. When revising a belief set K by some descriptor Ψ, the outcome K∘ Ψ is the belief set satisfying Ψ that is closest to K, as measured with δ. If we revise K∘ Ψ by Ξ, then the outcome K∘ Ψ ∘ Ξ is the belief set δ -closest to K∘ Ψ that satisfies Ξ, etc. The chapter also provides a generalization of blockage revision to global operations.
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