intuition.
eapply ratMult_inverse.
rewrite H.
eapply ratMult_eqRat_compat.
intuition.
eapply eqRat_terms; eauto.
Qed.

Lemma ratMult_same_r_inv : forall r1 r2 r3,
r1 * r2 == r3 * r2 ->
~ r2 == 0 ->
r1 == r3.