p = (struct bst_node *) &tree->bst_root;
for (cmp = -1; cmp != 0;
     cmp = tree->bst_compare (item, p->bst_data, tree->bst_param))
  {
    dir = cmp > 0;
    q = p;
    p = p->bst_link[dir];
    if (p == NULL)
      return NULL;
  }
item = p->bst_data;