intuition.
eapply mult_same_r; eauto.
rewrite mult_comm.
rewrite (mult_comm n2 n3).
trivial.
Qed.

(* Specifications: *)
Lemma mult_gt_0 : forall n1 n2,
n1 > 0 ->
n2 > 0 ->
n1 * n2 > 0.