Hint Resolve exp_l1.

(* Specifications: *)
Lemma exp_l2 :
forall (a : U) (n : Z), In U G a -> star a (exp n a) = exp (succZ n) a.
Lemma exp_l2' :
forall (a : U) (n : Z), In U G a -> star (inv a) (exp n a) = exp (predZ n) a.