return new OrderedPermutationCollection<E>(elements, comparator);
  }

  private static final class OrderedPermutationCollection<E> extends AbstractCollection<List<E>> {
    final ImmutableList<E> inputList;
    final Comparator<? super E> comparator;
    final int size;

    OrderedPermutationCollection(Iterable<E> input, Comparator<? super E> comparator) {