intros H'; try assumption.
red in H'.
simpl in H'.
inversion H'.
Qed.
Hint Resolve ax4: algebra.

Lemma Zl3 :
forall p : positive,
Equal (Z_to_group_nat_fun (Zneg p))
(group_inverse G (nat_to_group (nat_of_P (pos_abs (ax3 p))))).