Ltac destruct_match H :=
match type of H with
| context [ match ?x with _ => _  end] => destruct x eqn:?
end.

Lemma typeof_app : forall (e1 e2 : expr typ func) t tvs tus, typeof_expr tus tvs (App e1 e2) = Some t -> exists t2, typeof_expr tus tvs e1 = Some (tyArr t2 t) /\ typeof_expr tus tvs e2 = Some (t2).