intuition.
eapply (Build_EqDec (fun a1 a2 => if (eqd a1 a2) then true else false)).
intuition.
destruct (eqd x y); trivial.
discriminate.
destruct (eqd x y); trivial.
congruence.
Qed.

Lemma vector_0 : forall (A : Set)(v : Vector.t A 0),
v = Vector.nil A.