Proof with simpl_env; eauto.
intros E U T1 T2 Ok WA WU.
inversion WA; subst.
pick fresh X.
rewrite (subst_tt_intro X)...
rewrite_env (map (subst_tb X U) empty ++ E).
eapply wf_typ_subst_tb...
Qed.
(* ********************************************************************** *)
(** * #<a name="oktwft"></a># Properties of [wf_env] and [wf_typ] *)

Lemma uniq_from_wf_env : forall E,
wf_env E ->
uniq E.