Definition ghost_fp_update (a b : ghost) :=
forall n c, joins (ghost_fmap (approx n) (approx n) a) c ->
joins (ghost_fmap (approx n) (approx n) b) c.
Instance ghost_fp_update_preorder: RelationClasses.PreOrder ghost_fp_update.

Lemma Own_dealloc: forall a, Own a |-- bupd emp.
Definition singleton {A} k (x : A) : list (option A) := repeat None k ++ Some x :: nil.