proposition: Lemma le_def (a b : U) : (a <= b) = (a + b == b).
proof: Proof. by rewrite leEsub le_def -rmorphD /= (inj_eq val_inj). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype choice order) mathcomp(ssrnat bigop ssralg)