unfold ratSubtract in *.
rattac.
arithNormalize.
inversion H; clear H; subst.
simpl.
rewrite <- minus_n_O.
do 2 arithSimplify.
Qed.

(* Specifications: *)
Lemma ratDistance_0_r_le : forall r d,
r <= d ->
ratDistance r 0 <= d.