Definition ghost_fp_update_ND a B :=
forall n c, joins (ghost_fmap (approx n) (approx n) a) c ->
exists b, B b /\ joins (ghost_fmap (approx n) (approx n) b) c.

Lemma Own_update_ND: forall a B, ghost_fp_update_ND a B ->
Own a |-- bupd (EX b : _, !!(B b) && Own b).