code: public static <T> ItemsUnion<T> getInstance(final Comparator<? super T> comparator) {
      return new ItemsUnion<T>(PreambleUtil.DEFAULT_K, comparator, null);
    }
docs: Create an instance of ItemsUnion with the default k
  @param <T> type of item
  @param comparator to compare items
  @return an instance of ItemsUnion