*       (they will not be used).
   *   <li>If {@code count} is 1, {@code sumOfSquaresOfDeltas} must be exactly 0.0 or {@link
   *       Double#NaN}.
   * </ul>
   */
  Stats(long count, double mean, double sumOfSquaresOfDeltas, double min, double max) {
    this.count = count;
    this.mean = mean;
    this.sumOfSquaresOfDeltas = sumOfSquaresOfDeltas;
    this.min = min;
    this.max = max;
  }

  /**

    checkState(count() > 1);
    if (isNaN(sumOfProductsOfDeltas)) {
      return NaN;
    }
    double xSumOfSquaresOfDeltas = xStats().sumOfSquaresOfDeltas();
    double ySumOfSquaresOfDeltas = yStats().sumOfSquaresOfDeltas();
    checkState(xSumOfSquaresOfDeltas > 0.0);
    checkState(ySumOfSquaresOfDeltas > 0.0);
    // The product of two positive numbers can be zero if the multiplication underflowed. We

    checkState(count() > 1);
    if (isNaN(sumOfProductsOfDeltas)) {
      return NaN;
    }
    double xSumOfSquaresOfDeltas = xStats.sumOfSquaresOfDeltas();
    double ySumOfSquaresOfDeltas = yStats.sumOfSquaresOfDeltas();
    checkState(xSumOfSquaresOfDeltas > 0.0);
    checkState(ySumOfSquaresOfDeltas > 0.0);
    // The product of two positive numbers can be zero if the multiplication underflowed. We