intuition.
unfold ratSubtract in *.
rattac.
rewrite mult_0_l in e.
apply mult_is_O in e.
intuition.
Qed.

Lemma ratSubtract_le_sum : forall r1 r2 d,
r2 <= r1 ->
ratSubtract r1 r2 <= d ->
r1 <= r2 + d.