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 gfield0_offset_in_range: forall t gf, legal_field0 t gf -> complete_legal_cosu_type t = true -> 0 <= gfield_offset t gf /\ gfield_offset t gf <= sizeof t.