proposition: Lemma concat_length {s1 s2 : string} : length (s1 ++ s2) = length s1 + length s2.
proof: Proof. induction s1. { reflexivity. } { simpl. rewrite IHs1; 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