proposition: Lemma example n m k : k <= n -> (n <= m) && (m <= k) -> n = k.
proof: Proof. move=> lekn /andP H; case: H => lenm lemk. Undo. move=> lekn /andP[lenm lemk]. Abort. Lemma leq_total m n : (m <= n) || (m >= n). Proof. rewrite -implyNb -ltnNge. apply/implyP. by apply: ltnW. Qed.
imports: mathcomp(all_ssreflect)