for (q = NULL, p = tree->pbst_root; p != NULL; q = p, p = p->pbst_link[dir])
  {
    int cmp = tree->pbst_compare (item, p->pbst_data, tree->pbst_param);
    if (cmp == 0)
      return &p->pbst_data;
    dir = cmp > 0;
  }