code: public ItemsSketch<T> downSample(final int newK) {
      final ItemsSketch<T> newSketch = ItemsSketch.getInstance(newK, comparator_);
      ItemsMergeImpl.downSamplingMergeInto(this, newSketch);
      return newSketch;
    }
docs: From an existing sketch, this creates a new sketch that can have a smaller value of K.
  The original sketch is not modified.
  
  @param newK the new value of K that must be smaller than current value of K.
  It is required that this.getK() = newK * 2^(nonnegative integer).
  @return the new sketch.