static void mol_delete_cb(Fl_Widget *w, void *v) {
  VMDApp *app = (VMDApp *)w->user_data();
  MolBrowser *browser = (MolBrowser *)v;
  ResizeArray<int> idlist;
  for (int i=0; i<browser->size(); i++) {
    if (browser->selected(i+1)) {
      idlist.append(app->molecule_id(i));
    }
  }
  for (int j=0; j<idlist.num(); j++)
    app->molecule_delete(idlist[j]);
}