proposition: Lemma ltrD x y z t : x < y -> z < t -> x + z < y + t.
proof: Proof. by move=> lxy lzt; rewrite ltr_leD // ltW. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)