Proof with simpl_env; eauto.
intros E e e' T Typ. generalize dependent e'.
induction Typ; intros e' Red; try solve [ inversion Red; subst; eauto ].
Case "typing_app".
inversion Red; subst...
SCase "red_abs".
destruct (typing_inv_abs _ _ _ _ Typ1 T1 T2) as [P1 [S2 [L P2]]].
apply sub_reflexivity...
pick fresh x.
destruct (P2 x) as [? ?]...
rewrite (subst_ee_intro x)...
rewrite_env (empty ++ E).
apply (typing_through_subst_ee T).
apply (typing_sub S2)...
rewrite_env (empty ++ x ~ bind_typ T ++ E).
apply sub_weakening...
eauto.
Case "typing_tapp".
inversion Red; subst...
SCase "red_tabs".
destruct (typing_inv_tabs _ _ _ _ Typ T1 T2) as [P1 [S2 [L P2]]].
apply sub_reflexivity...
pick fresh X.
destruct (P2 X) as [? ?]...
rewrite (subst_te_intro X)...
rewrite (subst_tt_intro X)...
rewrite_env (map (subst_tb X T) empty ++ E).
apply (typing_through_subst_te T1)...
Case "typing_let".
inversion Red; subst.
SCase "red_let_1".
eapply typing_let; eauto.
SCase "red_let".
pick fresh x.
rewrite (subst_ee_intro x)...
rewrite_env (empty ++ E).
apply (typing_through_subst_ee T1)...
Case "typing_case".
inversion Red; subst.
SCase "red_case_1".
eapply typing_case; eauto.
SCase "red_case_inl".
destruct (typing_inv_inl _ _ _ Typ T1 T2) as [S1 [J2 J3]].
apply sub_reflexivity...
pick fresh x.
rewrite (subst_ee_intro x)...
rewrite_env (empty ++ E).
apply (typing_through_subst_ee T1)...
SCase "red_case_inr".
destruct (typing_inv_inr _ _ _ Typ T1 T2) as [S1 [J2 J3]].
apply sub_reflexivity...
pick fresh x.
rewrite (subst_ee_intro x)...
rewrite_env (empty ++ E).
apply (typing_through_subst_ee T2)...
Qed.
(* ********************************************************************** *)
(** * #<a name="progress"></a># Progress *)
(* ********************************************************************** *)
(** ** Canonical forms (14) *)

Lemma canonical_form_sum : forall e T1 T2,
value e ->
typing empty e (typ_sum T1 T2) ->
exists e1, e = exp_inl e1 \/ e = exp_inr e1.