(* ********************************************************************** *)
(** * #<a name="okt"></a># Properties of [wf_env] *)
(** These properties are analogous to the properties that we need to show for the subtyping and typing relations. *)

(* Specifications: *)
Lemma wf_env_narrowing : forall V E F U X,
wf_env (F ++ X ~ bind_sub V ++ E) ->
wf_typ E U ->
wf_env (F ++ X ~ bind_sub U ++ E).