*/
  public static <E> Collection<List<E>> permutations(Collection<E> elements) {
    return new PermutationCollection<E>(ImmutableList.copyOf(elements));
  }

  private static final class PermutationCollection<E> extends AbstractCollection<List<E>> {
    final ImmutableList<E> inputList;

    PermutationCollection(ImmutableList<E> input) {
      this.inputList = input;
    }

    @Override