proposition: Lemma lexU x y z : (x <= join y z) = (x <= y) || (x <= z).
proof: Proof. by rewrite joinEtotal le_max. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)