proposition: Lemma substring_substring {s n m n' m'} : substring n m (substring n' m' s) =s substring (n + n') (min m (m' - n)) s.
proof: Proof. rewrite !drop_take, !take_take, !drop_drop; 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