intros.
assert (X := @exprD'_typeof_expr typ _ _ _ _ _ _ _ tus (App e1 e2) tvs
t v).
assert (H2 : typeof_expr tus tvs (App e1 e2) = Some t).
apply X. intuition. clear X.
apply typeof_app; eauto.
Qed.

(* Specifications: *)
Lemma exprD'_one_type : forall tus tvs t1 t2 (e : expr typ func) v1 v2,
exprD' tus tvs t1 e = Some v1 ->
exprD' tus tvs t2 e = Some v2 ->
t1 = t2.