intuition.
eapply leRat_trans.
eapply ratSubtract_leRat_l; eauto.
eapply ratSubtract_leRat_r; eauto.
Qed.

(* Specifications: *)
Lemma ratSubtract_0 : forall r1 r2,
r1 <= r2 ->
ratSubtract r1 r2 == 0.