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 opt_eq_inv : forall (A : Type)(a1 a2 : A), Some a1 = Some a2 -> a1 = a2.