Proof using.
intros P Q eP eQ s1 s2 e un1; genclear e; genclear s2.
induction un1.
- intros s2 e; apply U0; apply eQ with s; assumption.
- intros (x2, s2) e.
apply U_next.
* apply eP with (Cons x s); assumption.
* apply IHun1.
case (exteq_inversion e). trivial.
Qed.

(* Specifications: *)
Lemma extensional_release :
forall (P Q: infseq T -> Prop),
extensional P -> extensional Q -> extensional (release P Q).