code: public ApproximateHistogram foldFast(ApproximateHistogram h, float[] mergedPositions, long[] mergedBins)
    {
      if (size == 0) {
        return copy(h);
      } else {
        return foldRule(h, mergedPositions, mergedBins);
      }
    }
docs: @param h               histogram to be merged into the current histogram
  @param mergedPositions temporary buffer of size greater or equal to {@link #size}
  @param mergedBins      temporary buffer of size greater or equal to {@link #size}
  
  @return returns this histogram with h folded into it