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