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

p = tree->tbst_root;
q = (struct tbst_node *) &tree->tbst_root;
dir = 0;
for (;;)
  {
    int cmp = tree->tbst_compare (item, p->tbst_data, tree->tbst_param);
    if (cmp == 0)
      break;

    dir = cmp > 0;
    if (p->tbst_tag[dir] == TBST_THREAD)
      return NULL;

    q = p;
    p = p->tbst_link[dir];
  }
item = p->tbst_data;