induction m; intuition.
rewrite (vector_0 v).
simpl.
trivial.
destruct (vector_S v).
destruct H.
subst.
simpl.
f_equal.
eapply IHm.
Qed.
Definition of_list_length (A : Set)(m : nat)(ls : list A)(pf : length ls = m) : Vector.t A m :=
match pf with
| eq_refl => Vector.of_list ls
end.
Definition of_sig_list (A : Set)(m : nat)(l : {ls : list A | length ls = m}) : Vector.t A m :=
match l with
| exist _ ls pf => (of_list_length ls pf)
end.

Lemma vector_hd_cons_eq : forall(A : Set)(v : Vector.t A 1),
v = Vector.cons _ (Vector.hd v) _ (Vector.nil A).