proposition: Lemma allLtn_consK b u a : allLtn (b :: u) a -> allLtn u a.
proof: Proof. Proof using. move/allP => Hall; apply/allP => x Hx; apply: Hall. by rewrite inE Hx orbT. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(order) tools