 The first rule, contraposition, for instance, says that if \(B\) is derivable from \(A\), then the negation of \(A\) is derivable from the negation of \(B\). All these rules and derivability statements are valid in classical logic (see the entry on  classical logic);  classical logic cannot distinguish between them. Some of these principles have been criticized and called into question in non-classical logic. The unrestricted and the negative ex contradictione rules, for example, introduce an element of irrelevancy because they allow to derive a completely arbitrary formula \(C\), respectively a completely arbitrary negated formula \(\osim C\), from an assumption \(A\) if a formula \(B\) as well as its negation \(\osim B\) are derivable from \(A\), see the entries on  relevance logic  and  paraconsistent logic.  Classical contraposition has been criticized because it gives rise to non-constructive existence proofs in languages containing the existential quantifier, see the entry on  intuitionistic logic.  In richer vocabularies, additional negation principles can be formulated, regimenting the interaction between negation and other logical operations. Prominent examples are the De Morgan Laws. In languages without implication, one may consider the following sequents stating De Morgan inference rules: