proposition: Lemma leZ_subRL m n p : (n <= p - m) <-> (m + n <= p).
proof: Proof. split => [/(@leZ_add2l m)|H]; first by rewrite subZKC. by apply (@leZ_add2l m); rewrite subZKC. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) ZArith Lia