proposition: Lemma singleton_exists_unique : forall s, length s = 1 -> exists !ch, s ~= [ ch ].
proof: Proof. intros s H'. destruct (singleton_exists s H') as [ch H'']. exists ch. split; [ apply H'' | ]. intro; apply singleton_unique; assumption. 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