Proof with auto.
induction v.
inversion i.
simpl.
destruct (natBelow_S_inv i).
destruct s.
subst.
simpl.
unfold natBelow_S_bla.
unfold Snb.
simpl.
destruct x.
apply IHv.
subst...
Qed.

Lemma nb1 (n: natBelow 1): n = mkNatBelow 0 0.