/* Returns the next data item in inorder
   within the tree being traversed with |trav|,
   or if there are no more data items returns |NULL|. */
void *
pbst_t_next (struct pbst_traverser *trav)
{
  assert (trav != NULL);

  if (trav->pbst_node == NULL)
    return pbst_t_first (trav, trav->pbst_table);
  else if (trav->pbst_node->pbst_link[1] == NULL)
    {
      struct pbst_node *q, *p; /* Current node and its child. */
      for (p = trav->pbst_node, q = p->pbst_parent; ;
           p = q, q = q->pbst_parent)
        if (q == NULL || p == q->pbst_link[0])
          {
            trav->pbst_node = q;
            return trav->pbst_node != NULL ? trav->pbst_node->pbst_data : NULL;
          }
    }
  else
    {
      trav->pbst_node = trav->pbst_node->pbst_link[1];
      while (trav->pbst_node->pbst_link[0] != NULL)
        trav->pbst_node = trav->pbst_node->pbst_link[0];
      return trav->pbst_node->pbst_data;
    }
}