(* not useful? *)

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