proposition: Lemma leBl z x y : x <= y -> x `\` z <= y `\` z.
proof: Proof. rewrite -{1}[x](joinIB z) -{1}[y](joinIB z). by rewrite leU2E ?meetIB ?meetBI // => /andP []. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)