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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?

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