proposition: Lemma widen_2: forall N1 N2, (measure (widen N1 N2) <= measure N1)%nat.
proof: Proof. intros. unfold widen. destruct (ble N2 N1) eqn:BLE. - lia. - destruct N1. cbn; lia. destruct N2; discriminate. Qed.
imports: Coq(ZArith Psatz Bool String List FMaps) Coq(FunctionalExtensionality) CDF(Sequences IMP)