if (tree->trb_root == NULL)
  return NULL;

p = (struct trb_node *) &tree->trb_root;
for (cmp = -1; cmp != 0;
     cmp = tree->trb_compare (item, p->trb_data, tree->trb_param))
  {
    dir = cmp > 0;
    pa[k] = p;
    da[k++] = dir;

    if (p->trb_tag[dir] == TRB_THREAD)
      return NULL;
    p = p->trb_link[dir];
  }
item = p->trb_data;