intuition.
unfold ratInverse.
rattac.
destruct n0.
exfalso.
eapply H.
eapply rat_num_0.
destruct n.
exfalso.
rewrite mult_0_l in l.
simpl in *.
remember (n0 * x1)%nat as a.
nia.
inversion H3; clear H3; subst.
inversion H2; clear H2; subst.
nia.
Qed.

Lemma ratAdd_not_leRat : forall r1 r2,
r1 + r2 <= r1 ->
(~r2 == 0) ->
False.