proposition: Lemma substring_concat_length {s1 s2 : string} : substring (length s1) (length s2) (s1 ++ s2) = s2.
proof: Proof. induction s1; simpl. { rewrite substring_correct3'; trivial. } { erewrite substring_correct4. { exact IHs1. } { rewrite concat_length; reflexivity. } { rewrite concat_length; reflexivity. } } 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