* elements but rowKeySet() will be empty and containsRow() won't
     * acknowledge them.
     */
    rowKeyToIndex = Maps.indexMap(rowList);
    columnKeyToIndex = Maps.indexMap(columnList);

    @SuppressWarnings("unchecked")
    @Nullable V[][] tmpArray = (@Nullable V[][]) new Object[rowList.size()][columnList.size()];
    array = tmpArray;

    PowerSet(Set<E> input) {
      checkArgument(
          input.size() <= 30, "Too many elements to create power set: %s > 30", input.size());
      this.inputSet = Maps.indexMap(input);
    }

    @Override
    public int size() {
      return 1 << inputSet.size();
    }

    @Override
    public boolean isEmpty() {
      return false;
    }

    }

    @Override
    public String toString() {
      return standardToString();
    }
  }

  /** Returns a map from the ith element of list to i. */
  static <E> ImmutableMap<E, Integer> indexMap(Collection<E> list) {
    ImmutableMap.Builder<E, Integer> builder = new ImmutableMap.Builder<>(list.size());
    int i = 0;
    for (E e : list) {
      builder.put(e, i++);
    }

    }

    @Override
    public String toString() {
      return standardToString();
    }
  }

  /** Returns a map from the ith element of list to i. */
  static <E> ImmutableMap<E, Integer> indexMap(Collection<E> list) {
    ImmutableMap.Builder<E, Integer> builder = new ImmutableMap.Builder<>(list.size());
    int i = 0;
    for (E e : list) {
      builder.put(e, i++);
    }