 The preceding sections have only dealt with changes of one and the same belief set or belief base. This is clearly a severe limitation. A realistic model of belief change should allow for repeated (iterated) changes, such as \(K\div p\div q* r* s\div t\ldots\) In other words, operations are needed that can contract or revise any belief set (belief base) by any sentence. Such operations are called global, in contrast to local operations that are defined only for a single specified set.