/* Initializes |trav| for |tree|
   and selects and returns a pointer to its least-valued item.
   Returns |NULL| if |tree| contains no nodes. */
void *
avl_t_first (struct avl_traverser *trav, struct avl_table *tree)
{
  struct avl_node *x;

  assert (tree != NULL && trav != NULL);

  trav->avl_table = tree;
  trav->avl_height = 0;
  trav->avl_generation = tree->avl_generation;

  x = tree->avl_root;
  if (x != NULL)
    while (x->avl_link[0] != NULL)
      {
        assert (trav->avl_height < AVL_MAX_HEIGHT);
        trav->avl_stack[trav->avl_height++] = x;
        x = x->avl_link[0];
      }
  trav->avl_node = x;

  return x != NULL ? x->avl_data : NULL;
}

