/* Copies tree rooted at |x| to |y|, which latter is allocated but not
   yet initialized. */
static void
bst_copy_recursive_2 (struct bst_node *x, struct bst_node *y)
{
  y->bst_data = x->bst_data;

  if (x->bst_link[0] != NULL)
    {
      y->bst_link[0] = malloc (sizeof *y->bst_link[0]);
      bst_copy_recursive_2 (x->bst_link[0], y->bst_link[0]);
    }
  else
    y->bst_link[0] = NULL;

  if (x->bst_link[1] != NULL)
    {
      y->bst_link[1] = malloc (sizeof *y->bst_link[1]);
      bst_copy_recursive_2 (x->bst_link[1], y->bst_link[1]);
    }
  else
    y->bst_link[1] = NULL;
}