intros e z H; case z; simpl in |- *; auto with real.
intros p; apply sym_eq; apply Rinv_involutive.
apply pow_nonzero; auto.
Qed.

Theorem powerRZ_Zs :
forall (e : R) (n : Z),
e <> 0%R -> powerRZ e (Zsucc n) = (e * powerRZ e n)%R.