/* Makes and returns a new copy of tree rooted at |x|. */
static struct bst_node *
bst_copy_recursive_1 (struct bst_node *x)
{
  struct bst_node *y;

  if (x == NULL)
    return NULL;

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

  y->bst_data = x->bst_data;
  y->bst_link[0] = bst_copy_recursive_1 (x->bst_link[0]);
  y->bst_link[1] = bst_copy_recursive_1 (x->bst_link[1]);
  return y;
}