Let $L$ is a lattice.
At http://planetmath.org/encyclopedia/Benzene.html it's written:
It is easy to see that given an element $a\in L$, the pseudocomplement of $a$, if it exists, is unique.
Is it true in general? I see a proof only for special classes of lattices, such as distributive lattice. Is it an error in PlanetMath or I just miss a proof for the general case?