static struct bst_node error_node;

/* Makes and returns a new copy of tree rooted at |x|.
   If an allocation error occurs, returns |&error_node|. */
static struct bst_node *
bst_robust_copy_recursive_2 (struct bst_node *x)
{
  struct bst_node *y;

  if (x == NULL)
    return NULL;

  y = malloc (sizeof *y);
  if (y == NULL)
    return &error_node;

  y->bst_data = x->bst_data;
  y->bst_link[0] = bst_robust_copy_recursive_2 (x->bst_link[0]);
  y->bst_link[1] = bst_robust_copy_recursive_2 (x->bst_link[1]);
  if (y->bst_link[0] == &error_node || y->bst_link[1] == &error_node)
    {
      bst_deallocate_recursive (y);
      return &error_node;
    }

  return y;
}