Proof using.
intros P eP s1 s2 e; destruct e; simpl. unfold not_tl.
intros P1 nP2.
contradict P1.
apply eP with (Cons x s2); trivial.
apply exteq_sym.
apply exteq_intro.
assumption.
Qed.

(* Specifications: *)
Lemma extensional_now :
forall (P: T -> Prop), extensional (now P).