Proof with auto.
induction n.
intros.
rewrite (eq_nil v)...
intros.
rewrite (eq_cons v).
simpl.
rewrite IHn...
Qed.

(* Specifications: *)
Lemma In_vec_inv A a n (v: Vector.t A n): List.In a v -> exists i, a = nth v i.
Proof with auto.