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 shiftOut_0 : forall (s : Blist), shiftOut s 0 = Some ([], s).