public String toString() {
      return "permutations(" + inputList + ")";
    }
  }

  private static final class PermutationIterator<E> extends AbstractIterator<List<E>> {
    final List<E> list;
    final int[] c;
    final int[] o;
    int j;

    PermutationIterator(List<E> list) {
      this.list = new ArrayList<>(list);
      int n = list.size();
      c = new int[n];
      o = new int[n];