(** ** [RegExp] as Setoid. *)
(** [re_eq] is an equivalence relation between [RegExp]. *)
Definition re_eq (re re':RegExp) : Prop := forall s, re ~= s = re' ~= s.
Notation "a =R= b" := (re_eq a b) (at level 70).

Lemma re_eq_sym : symmetric RegExp re_eq.