simple induction n2.
(* O *)
intros; elim (addZ_commutativity (neg 0) (pos n1)).
rewrite addZ_eq4.
elim minus_n_Sm; trivial.
elim minus_n_O.
apply tech_pred_posZ; trivial.
(* S y *)
intros; elim (addZ_commutativity (neg (S n)) (pos n1)).
rewrite tech_add_neg_predZ.
elim (addZ_commutativity (pos n1) (neg n)).
rewrite H; auto with arith.
elim (minus_n_Sm n1 (S n) H0).
apply tech_pred_posZ.
apply lt_minus2; trivial.
Qed.

(*************************)
Theorem addZ_associativity : associativity Z addZ.
unfold associativity in |- *; intros; elim x.