Proof with eauto using wf_typ_strengthening, wf_env_strengthening.
intros x U E F S T SsubT.
remember (F ++ x ~ bind_typ U ++ E) as E'.
generalize dependent F.
induction SsubT; intros F EQ; subst...
Case "sub_trans_tvar".
apply (sub_trans_tvar U0)...
analyze_binds H...
Case "sub_all".
pick fresh X and apply sub_all...
rewrite <- app_assoc.
apply H0...
Qed.
(************************************************************************ *)
(** ** Narrowing for typing (7) *)

Lemma typing_narrowing : forall Q E F X P e T,
sub E P Q ->
typing (F ++ X ~ bind_sub Q ++ E) e T ->
typing (F ++ X ~ bind_sub P ++ E) e T.