static void
traverse_iterative (struct bst_node *node, bst_item_func *action, void *param)
{
  struct bst_node **stack = NULL;
  size_t height = 0;
  size_t max_height = 0;

  for (;;)
    {
      while (node != NULL)
        {
          if (height >= max_height)
            {
              max_height = max_height * 2 + 8;
              stack = realloc (stack, sizeof *stack * max_height);
              if (stack == NULL)
                {
                  fprintf (stderr, "out of memory\n");
                  exit (EXIT_FAILURE);
                }
            }

          stack[height++] = node;
          node = node->bst_link[0];
        }

      if (height == 0)
        break;

      node = stack[--height];
      action (node->bst_data, param);
      node = node->bst_link[1];
    }

  free (stack);
}