intuition.
eapply leRat_impl_eqRat.
eapply leRat_ratAdd_same_r;
eapply eqRat_impl_leRat; eauto.
symmetry in H.
eapply leRat_ratAdd_same_r;
eapply eqRat_impl_leRat; eauto.
Qed.

Lemma ratAdd_arg_0 : forall a b,
a + b == a ->
b == 0.