intuition.
unfold ratHalf in *.
rattac.
unfold posnatMult in *.
destruct p.
unfold natToPosnat in *.
unfold posnatToNat in *.
rewrite mult_1_r in *.
rewrite (mult_comm x) in l.
rewrite mult_assoc in *.
assert (n * 2 <= n)%nat.
eapply mult_le_compat_r_iff; eauto.
omega.
Qed.

Lemma ratSubtract_0_r : forall r,
ratSubtract r 0 == r.