code: public static <T> ItemsUnion<T> getInstance(final int maxK, final Comparator<? super T> comparator) {
      return new ItemsUnion<>(maxK, comparator, null);
    }
docs: Create an instance of ItemsUnion
  @param <T> type of item
  @param maxK determines the accuracy and size of the union and is a maximum value.
  The effective <i>k</i> can be smaller due to unions with smaller <i>k</i> sketches.
  It is recommended that <i>maxK</i> be a power of 2 to enable unioning of sketches with
  different values of <i>k</i>.
  @param comparator to compare items
  @return an instance of ItemsUnion