rewrite mult_comm.
induction m.
inversion H0.
simpl.
assert (O < m*n);try omega.
inversion H0;try omega.
assert (1 <= n);try omega.
assert (m > 1);try omega.
generalize (IHm H4);omega.
Qed.

Lemma mult_lemma4 : forall (n m:nat),n=n*m -> n=O \/ m=1.
intros n m.