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_nested_field: forall t gfs p, field_compatible t gfs p -> field_compatible (nested_field_type t gfs) nil (offset_val (nested_field_offset t gfs) p).