 The two major tasks of an operation \(*\) of revision are (1) to add the new belief \(p\) to the belief set \(K\), and (2) to ensure that the resulting belief set \(K* p\) is consistent (unless \(p\) is inconsistent). The first task can be accomplished by expansion by \(p\). The second can be accomplished by prior contraction by its negation \(\neg p\). If a belief set does not imply \(\neg p\), then \(p\) can be added to it without loss of consistency. This composition of suboperations gives rise to the following definition of revision (Gärdenfors 1981, Levi 1977):