 Put
Then there are congruences formula_19, for "j<2", such that

The proof of the theorem above runs by setting a "structure" theorem for congruence lattices of semilattices—namely, the Erosion Lemma, against "non-structure" theorems for free distributive extensions "G(Ω)", the main one being called the "Evaporation Lemma".While the latter are technically difficult, they are, in some sense, predictable.Quite to the contrary, the proof of the Erosion Lemma is elementary and easy, so it is probably the strangeness of its statement that explains that it has been hidden for so long.