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_field_address: forall t gfs p, field_compatible t gfs p -> field_address t gfs p = offset_val (nested_field_offset t gfs) p.