code: private void twoWayMergeInternalStandard(final ReservoirItemsSketch<T> source) {
      assert (source.getN() <= source.getK());
      final int numInputSamples = source.getNumSamples();
      for (int i = 0; i < numInputSamples; ++i) {
        gadget_.update(source.getValueAtPosition(i));
      }
    }
docs: should be called ONLY by twoWayMergeInternal