template <typename T>
Node<T>* sort2n(Node<T>** x, int n) {
  // Take a single element
  Node<T>* sorted = take1(x);

  for (int i = 0; *x && i < n; ++i) {
    // Merge in 2^i sorted elements.
    merge(&sorted, sort2n(x, i));
  }

  return sorted;
}