Definition Blist := list bool.Definition Blist_eq_dec := (list_eq_dec bool_dec).Definition Bvector_eq_dec(n : nat)(v1 v2 : Bvector n) : {v1 = v2} + {v1 <> v2}.(* shiftOut gets bits from the head of the list. *)
(* return None when we run out *)
Fixpoint shiftOut(s : Blist)(n : nat) : option (Bvector n * Blist) :=
  match n with
    | 0 => Some ((@Vector.nil bool), s)
    | S n' => 
      match s with
        | nil => None
        | b :: s' => 
          match (shiftOut s' n') with
            | Some (v', s'') => Some (Vector.cons _ b _ v', s'')
            | None => None
          end
      end
  end.

Theorem shiftOut_None_inv : forall n ls, shiftOut ls n = None -> n > length ls.