intros; elim m.
(* O *)
elim multZ_neutral; intros; elim (H0 (pos n) I); intros.
replace (pos 0) with IZ; auto.
rewrite H1.
elim (mult_commut 1 (S n)). rewrite (mult_neutr (S n)).
unfold posOZ in |- *; reflexivity.
(* S y *)
intros y H; elim (multZ_commutativity (pos (S y)) (pos n)).
rewrite (tech_mult_posZ y (pos n));
elim (multZ_commutativity (pos n) (pos y)).
elim H; elim (mult_n_Sm (S n) (S y)); elim (plus_n_Sm (S n * S y) n).
elim (mult_n_Sm (S n) y); elim (plus_n_Sm (S n * y) n).
unfold posOZ in |- *; rewrite (tech_add_pos_posZ (S n * y + n) n).
reflexivity.
Qed.

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Lemma tech_mult_pos_succZ2 :
forall n m : nat, multZ (pos n) (pos m) = pos (S n * m + n).