proposition: Lemma minus_le a b: a <= b -> a - b = zero.
proof: Proof. rewrite T1le_eqVlt;case /orP; [move /eqP ->; apply: T1subnn| apply: minus_lt]. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)