intuition.
rattac.
symmetry.
eapply le_n_0_eq.
unfold natToPosnat, posnatToNat in *.
rewrite mult_1_r in l.
rewrite l.
destruct p.
rewrite mult_0_l.
intuition.
Qed.

Lemma rat_le_1_if :
forall n d,
RatIntro n d <= 1 ->
(n <= d)%nat.