Proof with simpl_env; eauto.
intros T E F G Hwf_typ Hk.
remember (G ++ E) as F'.
generalize dependent G.
induction Hwf_typ; intros G Hok Heq; subst...
Case "type_all".
pick fresh Y and apply wf_typ_all...
rewrite <- app_assoc.
apply H0...
Qed.

Lemma wf_typ_narrowing : forall V U T E F X,
wf_typ (F ++ X ~ bind_sub V ++ E) T ->
wf_typ (F ++ X ~ bind_sub U ++ E) T.