  void fill_with_frame_segs(ES_list& L, const Tag_true& ) const
  /*{\Xop fills the list with the four segments which span our frame,
     the convex hull of SW,SE,NW,NE.}*/
  { L.push_back(Extended_segment(EK.SW(),EK.NW()));
    L.push_back(Extended_segment(EK.SW(),EK.SE()));
    L.push_back(Extended_segment(EK.NW(),EK.NE()));
    L.push_back(Extended_segment(EK.SE(),EK.NE()));
  }
  void fill_with_frame_segs(ES_list& L, const Tag_true& ) const
   
  { L.push_back(Extended_segment(EK.SW(),EK.NW()));
    L.push_back(Extended_segment(EK.SW(),EK.SE()));
    L.push_back(Extended_segment(EK.NW(),EK.NE()));
    L.push_back(Extended_segment(EK.SE(),EK.NE()));
  }