proposition: Lemma leBLR x y z : (x `\` y <= z) = (x <= y `|` z).
proof: Proof. apply/idP/idP; first by move=> /join_r <-; rewrite joinA diffKU joinAC leUr. by rewrite -{1}[x](joinIB y) => /(leU2r_le (diffIK _ _)). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)