proposition: Theorem le_1_r : forall n, n <= 1 <-> n == 0 \/ n == 1.
proof: Proof. setoid_rewrite one_succ. intro n; cases n. - split; intro; [now left | apply le_succ_diag_r]. - intro n. rewrite <- succ_le_mono, le_0_r, succ_inj_wd. split; [intro; now right | intros [H | H]; [false_hyp H neq_succ_0 | assumption]]. Qed.
imports: NAdd