rewrite (vec.eq_cons v).
simpl.
reflexivity.
Qed.

Lemma rect (Q: list X -> M (list X) -> Type):
(Q nil (ret nil)) ->
(forall n (v: Vector.t X (S n)), (forall y, length y < S n -> Q y (qs cmp pick y)) -> Q v (selectPivotPart v)) ->
forall x, Q x (qs cmp pick x).