proposition: Lemma subr_lt0 x y : (y - x < 0) = (y < x).
proof: Proof. by rewrite -[LHS]subr_gt0 opprB add0r subr_gt0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)