proposition: Lemma first_char_in_singleton_str (str : String) ls ch (H : str ~= [ ch ]) : first_char_in str ls <-> List.In ch ls.
proof: Proof. unfold first_char_in. rewrite <- for_first_char_singleton; try eassumption; reflexivity. Qed.
imports: Fiat.Common.Coq__8_4__8_5__Compat Coq.ZArith.ZArith Fiat.Parsers.StringLike.Core Fiat.Parsers.StringLike.Properties Fiat.Parsers.StringLike.ForallChars Fiat.Common