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 le_refl_gen : forall n1 n2, (n1 = n2 -> n1 <= n2)%nat.