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