code: public static <T> ItemsUnion<T> getInstance(final ItemsSketch<T> sketch) {
      return new ItemsUnion<>(sketch.getK(), sketch.getComparator(), ItemsSketch.copy(sketch));
    }
docs: Create an instance of ItemsUnion based on ItemsSketch
  @param <T> type of item
  @param sketch the basis of the union
  @return an instance of ItemsUnion