proposition: Lemma ltIx x y z : (meet y z < x) = (y < x) || (z < x).
proof: Proof. by rewrite !ltNge lexI negb_and. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)