(** ** Define [includes] *)

Definition includes (x y:RegExp) : Prop := (x || y) =R= y.Notation "x <= y" := (includes x y) (at level 70).Definition prop_eq (p q:Prop) : Prop := (p <-> q).

Theorem Star_eq_left : forall a b x, (b || (a ++ x)) <= x -> ((Star a) ++ b) <= x.