unfold ratSubtract, ratAdd.
rattac.
arithNormalize.
generalize (n1 * x0 * x1 * x * x0 * x1)%nat; intuition.
assert (n3 * x * x1 * x * x0 * x1 = n3 * x1 * x * x * x0 * x1)%nat.
do 5 arithSimplify.
rewrite H.
generalize (n3 * x1 * x * x * x0 * x1)%nat; intuition.
assert (n0 * x * x0 * x * x0 * x1 = n0 * x0 * x * x * x0 * x1)%nat.
do 5 arithSimplify.
rewrite H0.
generalize ( n0 * x0 * x * x * x0 * x1)%nat; intuition.
Qed.

Lemma ratSubtract_assoc_le : forall r1 r2 r3,
ratSubtract r1 (ratSubtract r2 r3) <= (ratSubtract r1 r2) + r3.