intuition.
induction n, v using Vector.rectS.
econstructor. econstructor. eauto.
econstructor. econstructor. eauto.
Qed.
Definition eqb_vector(A : Set)(eqd : EqDec A)(n : nat)(v1 v2 : Vector.t A n) :=
Vector.fold_left2 (fun b a1 a2 => b && (eqb a1 a2)) true v1 v2.

Theorem eqb_vector_refl :
forall (A : Set)(eqd : EqDec A)(n : nat)(v : Vector.t A n),
eqb_vector _ v v = true.