(* OZ *)
split; reflexivity.
(* pos n *)
simple induction n.
(* pos O *)
split.
rewrite addZ_eq2; rewrite multZ_eq2.
rewrite (mult_succZ_l y z); exact (addZ_commutativity (multZ y z) z).
reflexivity.
(* pos (S y0) *)
intros y0 H.
elim H; intros; split.
rewrite addZ_eq3; rewrite multZ_eq3.
rewrite mult_succZ_l; rewrite H0.
elim (addZ_associativity (multZ (pos y0) z) (multZ y z) z).
elim (addZ_commutativity z (multZ y z)).
apply addZ_associativity.
do 3 rewrite multZ_eq3.
rewrite H1.
apply (add_add Z addZ addZ_commutativity addZ_associativity).
(* neg n *)
simple induction n.
(* neg O *)
split.
rewrite addZ_eq4; rewrite multZ_eq4; rewrite (mult_predZ_l y z).
exact (addZ_commutativity (multZ y z) (oppZ z)).
rewrite multZ_eq4.
apply (opp_add Z IdZ addZ OZ oppZ Z_group addZ_commutativity y z I I).
(* neg (S y0) *)
intros y0 H.
split.
(* -> *)
rewrite (tech_add_neg_predZ y0 y); rewrite (mult_predZ_l (addZ (neg y0) y) z).
elim H; intros. rewrite H0.
elim (addZ_associativity (multZ (neg y0) z) (multZ y z) (oppZ z)).
elim (addZ_commutativity (oppZ z) (multZ y z)).
rewrite (addZ_associativity (multZ (neg y0) z) (oppZ z) (multZ y z)).
elim (tech_mult_negZ y0 z); reflexivity.
(* <- *)
rewrite (tech_mult_negZ y0 (addZ y z)); rewrite (tech_mult_negZ y0 y).
rewrite (tech_mult_negZ y0 z); elim H; intros; rewrite H1.
elim
(add_add Z addZ addZ_commutativity addZ_associativity
(multZ (neg y0) y) (multZ (neg y0) z) (oppZ y)
(oppZ z)).
elim (opp_add Z IdZ addZ OZ oppZ Z_group addZ_commutativity y z I I).
reflexivity.
Qed.

(****************)
Lemma mult_oppZ_r : forall x y : Z, multZ x (oppZ y) = oppZ (multZ x y).
intros; case x.