Heyting algebra
Heyting algebras are an example of distributive lattices where some members might be lacking complements. Every element "x" of a Heyting algebra has, on the other hand, a pseudo-complement, also denoted ¬"x". The pseudo-complement is the greatest element "y" such that . If the pseudo-complement of every element of a Heyting algebra is in fact a complement, then the Heyting algebra is in fact a Boolean algebra.