Proof with eauto using subst_tt_type.
intros Z P e He Hp.
induction He; simpl; auto using subst_tt_type;
try solve [
econstructor;
try instantiate (1 := L `union` singleton Z);
intros;
try rewrite subst_te_open_ee_var;
try rewrite subst_te_open_te_var;
instantiate;
eauto using subst_tt_type
].
Qed.
(** The following lemma depends on [subst_ee_open_ee_var] and [subst_ee_open_te_var]. *)

(* Specifications: *)
Lemma subst_ee_expr : forall z e1 e2,
expr e1 ->
expr e2 ->
expr (subst_ee z e2 e1).