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 BVxor_same_id : forall n (v : Bvector n), BVxor n v v = Bvect_false n.