Fixpoint list_denote l : T :=
  match l with nil => b | x :: l' => g (f x) (list_denote l')
  end.Notation "'un'" := (@orp _).

Lemma listd_merge_un' l1 l2 s : listd f un FF (merge cmp l1 l2) s -> listd f un FF l1 s \/ listd f un FF l2 s.