  typename EK::Plane_3 read_plane_impl(std::istream& in, Null_functor) {
    typename K::FT a, b, c, d;
    in >> a >> b >> c >> d;
    return typename EK::Plane_3(a,b,c,d);
  }
  typename EK::Plane_3 read_plane_impl(std::istream& in, Null_functor) {
    typename K::FT a, b, c, d;
    in >> a >> b >> c >> d;
    return typename EK::Plane_3(a,b,c,d);
  }