proposition: Lemma leBUK y x : y <= x -> (x `\` y) `|` y = x.
proof: Proof. by move=> leyx; rewrite joinC leBKU. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)