 Another example is Dunn's 4-valued logic, in which "false" < "neither-true-nor-false" < "true" and "false" < "both-true-and-false" < "true", while "neither-true-nor-false" and "both-true-and-false" are not comparable.If a De Morgan algebra additionally satisfies "x" ∧ ¬"x" ≤ "y" ∨ ¬"y", it is called a Kleene algebra.