  void extract_closure()
  /*{\Xop converts |\Mvar| to its closure. }*/
  { CGAL_NEF_TRACEN("extract closure");
    extract_complement();
    extract_interior();
    extract_complement();
  }
  void extract_closure()
   
  { CGAL_NEF_TRACEN("extract closure");
    extract_complement();
    extract_interior();
    extract_complement();
  }