    void supporting_segment(Halfedge_handle e, ES_iterator it)
    {
      if ( it == _it )
        _e = e;
      D.mark(e) = _m;
    }
    void supporting_segment(Halfedge_handle e, ES_iterator it)
    {
      if ( it == _it )
        _e = e;
      D.mark(e) = _m;
    }