(* not useful? *)

Lemma mapsto_field_at_ramify {cs: compspecs} sh t gfs v v' w w' p:
type_is_by_value (nested_field_type t gfs) = true ->
type_is_volatile (nested_field_type t gfs) = false ->
JMeq v v' ->
JMeq w w' ->
field_at sh t gfs v' p |--
mapsto sh (nested_field_type t gfs) (field_address t gfs p) v *
(mapsto sh (nested_field_type t gfs) (field_address t gfs p) w -*
field_at sh t gfs w' p).