bool selected(k3d::face* face)
  {
   if (face->visible_selection)
    return true;
   k3d::split_edge* edge = face->first_edge;
   do {
    if (edge->visible_selection || edge->vertex->visible_selection || (edge->companion && (edge->companion->visible_selection || edge->companion->vertex->visible_selection)))
     return true;
   } while (edge != face->first_edge);
   return false;
  }