intuition.
unfold posnatMult.
destruct p.
remember (pos 1) as p2.
destruct p2.
unfold posnatToNat.
inversion Heqp2; subst.
apply mult_1_r.
Qed.

Lemma rat_num_nz : forall n d,
n > 0 ->
RatIntro n d == 0 ->
False.