Proof Z.div_unique.

Lemma Zdiv_0_r: forall a, a/0 = 0.
Ltac zero_or_not a :=
destruct (Z.eq_dec a 0);
[subst; rewrite ?Zmod_0_l, ?Zdiv_0_l, ?Zmod_0_r, ?Zdiv_0_r;
auto with zarith|].