(** ** String *)
(** For simplicity, there is no [RegExp] constructor for string; however, the conversion is easy. *)

(* Specifications: *)
Lemma string_to_re_true : forall s:string, (string_to_re s) ~== s.
Lemma string_to_re_false : forall s s':string, s <> s' -> (string_to_re s) ~!= s'.