proposition: Lemma minus_lt a b: a < b -> a - b = zero.
proof: Proof. elim: a b => // a' _ n' b' Hb [] // a'' n'' b'' h. by rewrite /T1sub h. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)