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 ext_nth A n (x y: Vector.t A n): ext_eq (nth x) (nth y) -> x = y.
Proof with auto.