 We put
and we denote by Conc"U" "L" the (∨,0)-subsemilattice of Conc "L" generated by all principal congruences Θ("u","v") ( = least congruence of "L" that identifies "u" and "v"), where ("u","v") belongs to "U" ×"U".We put Θ+("u","v")=Θ("u ∨ v","v"), for all "u, v" in "L".br />

The Erosion Lemma (Wehrung 2007).Let "x"0, "x"1 in "L" and let formula_16, for a positive integer "n", be a finite subset of "L" with formula_17.