elim (mult_reg_l a b p); auto with arith.
intro; absurd (p = 0); auto with arith.
Qed.
Hint Immediate mult_reg_l_bis.

(* Specifications: *)
Lemma mult_eq_zero_bis : forall a b : nat, 0 < a -> a * b = 0 -> b = 0.
intros a b pos H.