proposition: Lemma ltr_leB x y z t : x < y -> t <= z -> x - z < y - t.
proof: Proof. by move=> lxy lzt; rewrite ltr_leD // lterN2. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)