Fixpoint tailOpt(A : Set)(n : nat)(v : Vector.t A n) : option (Vector.t A (pred n)):=
  match v with
    | [] => None
    | Vector.cons _ _ _ v => Some v
  end.

Lemma shiftOut_app_None : forall ls1 ls2 n, shiftOut (ls1 ++ ls2) n = None -> shiftOut ls1 n = None.