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 field_compatible_range: forall i lo hi t gfs p, lo <= i < hi -> field_compatible0 t (ArraySubsc lo :: gfs) p -> field_compatible0 t (ArraySubsc hi :: gfs) p -> field_compatible t (ArraySubsc i :: gfs) p.