proposition: Lemma substring_of_string {n m str} : substring n m (of_string str) =s of_string (List.take m (List.drop n str)).
proof: Proof. rewrite <- take_of_string, <- drop_of_string; reflexivity. Qed.
imports: Fiat.Common.Coq__8_4__8_5__Compat Coq.ZArith.ZArith Fiat.Common Fiat.Common.List.Operations Fiat.Common.List.ListFacts Fiat.Common.Le Fiat.Common.Tactics.SetoidSubst Fiat.Common.Tactics.IsClosed Fiat.Parsers.StringLike.Core Fiat.Common.UIP