rattac.
unfold posnatToNat, natToPosnat in *.
destruct d.
simpl in *.
omega.
Qed.

Lemma ratMult_inverse : forall r1 r2 (p1 p2 : posnat),
r1 == r2 * (RatIntro p1 p2) ->
r1 * (RatIntro p2 p1) == r2.