proposition: Lemma leBr z x y : x <= y -> z `\` y <= z `\` x.
proof: Proof. by move=> lexy; rewrite leBLR joinxB meet_r ?leBUK ?leUr ?lexUl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)