intuition.
eapply eqRat_trans.
eapply ratAdd_eqRat_compat_l; eauto.
repeat rewrite (ratAdd_comm r2).
eapply ratAdd_eqRat_compat_l; eauto.
Qed.

Lemma ratAdd_leRat_compat_l : forall r1 r2 r3,
leRat r1 r2 ->
r1 + r3 <= r2 + r3.