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 legal_nested_field0_shrink1: forall t gfs0 gfs1, gfs1 <> nil -> legal_nested_field0 t (gfs1 ++ gfs0) -> legal_nested_field t gfs0.