(* OZ *)
reflexivity.
(* pos n *)
simple induction n.
(* pos O *)
unfold multZ in |- *; reflexivity.
(* pos (S y0) *)
intros y0 H; do 2 rewrite (tech_mult_posZ y0).
rewrite H; elim (mult_oppZ_l y z).
elim (mult_add_distributivity (multZ (pos y0) y) y z); intros. elim H0.
reflexivity.
(* neg n *)
simple induction n.
(* neg O *)
simpl in |- *; symmetry  in |- *; exact (mult_oppZ_l y z).
(* neg (S y0) *)
intros y0 H; do 2 rewrite (tech_mult_negZ y0).
rewrite H; elim (mult_oppZ_l y z).
elim (mult_add_distributivity (multZ (neg y0) y) (oppZ y) z); intros. elim H0.
reflexivity.
Qed.

(*************)
Theorem Z_ring : is_ring Z IdZ addZ multZ OZ oppZ.
unfold is_ring in |- *.