}

  private Item<T> getItem(int groupNumber, int itemNumber) {
    return new Item<>(groups.get(groupNumber).get(itemNumber), groupNumber, itemNumber);
  }

  static final class Item<T> {
    final T value;
    final int groupNumber;
    final int itemNumber;

    Item(T value, int groupNumber, int itemNumber) {
      this.value = value;
      this.groupNumber = groupNumber;
      this.itemNumber = itemNumber;