 The construction of package revision gives rise to an interesting extension of the notion of negation. The reason why contraction by \(\neg p\) is performed as a suboperation of revision by \(p\) is that \(p\) can be consistently added to a set if and only if it does not imply \(\neg p\). It turns out that (in a logic satisfying compactness) a set \(B\) can be consistently added to another set if and only if this other set does not imply any element of the set \(\neg B\), that is defined as follows: