proposition: Lemma substring_correct0' {s : string} {n m} (H : length s <= n) : substring n m s = ""%string.
proof: Proof. revert n m H; induction s; simpl; intros n m H. { destruct n, m; trivial. } { destruct n, m; trivial. { apply Nat.nlt_0_r in H; destruct H. } { rewrite IHs; eauto with arith. } { rewrite IHs; eauto with arith. } } Qed.
imports: Fiat.Common.Coq__8_4__8_5__Compat Coq.ZArith.ZArith Coq.Arith.PeanoNat Coq.Strings.Ascii Coq.Strings.String Fiat.Common.List.Operations Fiat.Common.StringOperations