unfold group_power in |- *; auto with algebra.
Qed.
Hint Resolve group_power_plus: algebra.

(* Specifications: *)
Lemma group_power_S :
forall (g : G) (n : ZZ),
Equal (group_power G g (sgroup_law ZZ n (ring_unit ZZ)))
(sgroup_law G (group_power G g n) g).