intuition.
destruct s; simpl in *; trivial.
Qed.

(* Specifications: *)
Theorem shiftOut_S_None : forall (n : nat)(s s1 : Blist)(v1 : Bvector 1),
shiftOut s 1 = Some (v1, s1) ->
shiftOut s1 n = None ->
shiftOut s (S n) = None.