Definition field_address t gfs p :=
  if (field_compatible_dec t gfs p)
  then offset_val (nested_field_offset t gfs) p
  else Vundef.Definition field_address0 t gfs p :=
  if (field_compatible0_dec t gfs p)
  then offset_val (nested_field_offset t gfs) p
  else Vundef.

Lemma nested_field_offset_in_range: forall t gfs, legal_nested_field t gfs -> complete_legal_cosu_type t = true -> 0 <= nested_field_offset t gfs /\ (nested_field_offset t gfs) + sizeof (nested_field_type t gfs) <= sizeof t.