proposition: Lemma substring_concat' {y z} {s : string} : (substring 0 y s ++ substring y z s)%string = substring 0 (y + z) s.
proof: Proof. rewrite <- substring_concat; 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