rect* rect::get_bbox(rect** rect_list,int num_rect) {
   double minx = (*rect_list)->x1();
   double miny = (*rect_list)->y1();
   double maxx = (*rect_list)->x2();
   double maxy = (*rect_list)->y2();
   int i;
   for (i=1;i<num_rect;i++) {
      double tmp_minx = (*(rect_list+i))->x1();
      double tmp_miny = (*(rect_list+i))->y1();
      double tmp_maxx = (*(rect_list+i))->x2();
      double tmp_maxy = (*(rect_list+i))->y2();
      if (tmp_minx < minx)
         minx = tmp_minx;
      if (tmp_maxx > maxx)
         maxx = tmp_maxx;
      if (tmp_miny < miny)
         miny = tmp_miny;
      if (tmp_maxy > maxy)
         maxy = tmp_maxy;
   }
   rect* result;
   rect r(minx,miny,maxx,maxy);
   *result = r;
   return (result);
}