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

p = tree->pbst_root;
for (;;)
  {
    int cmp = tree->pbst_compare (item, p->pbst_data, tree->pbst_param);
    if (cmp == 0)
      break;

    dir = cmp > 0;
    p = p->pbst_link[dir];
    if (p == NULL)
      return NULL;
  }
item = p->pbst_data;

q = p->pbst_parent;
if (q == NULL)
  {
    q = (struct pbst_node *) &tree->pbst_root;
    dir = 0;
  }