  Nef_polyhedron_2<T,Items,Mark> complement() const
  /*{\Mop returns the complement of |\Mvar| in the plane.}*/
  { Nef_polyhedron_2<T,Items,Mark> res = *this;
    res.extract_complement();
    return res;
  }
  Nef_polyhedron_2<T,Items,Mark> complement() const
   
  { Nef_polyhedron_2<T,Items,Mark> res = *this;
    res.extract_complement();
    return res;
  }