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_update: forall a b, ghost_fp_update a b ->
Own a |-- bupd (Own b).