Proof with eauto.
intros E T Ok Wf.
induction Wf...
pick fresh Y and apply sub_all...
Qed.
(* ********************************************************************** *)
(** ** Weakening (2) *)

(* Specifications: *)
Lemma sub_weakening : forall E F G S T,
sub (G ++ E) S T ->
wf_env (G ++ F ++ E) ->
sub (G ++ F ++ E) S T.