// from https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm
srecord Welford(long count, dbl mean, dbl m2) {
  // For a new value newValue, compute the new count, new mean, the new M2.
  // mean accumulates the mean of the entire dataset
  // M2 aggregates the squared distance from the mean
  // count aggregates the number of samples seen so far
  void add(double newValue) {
    count++;
    dbl delta = newValue - mean;
    mean += delta / count;
    db ldelta2 = newValue - mean
    m2 += delta * delta2;
  }

  dbl mean() { ret count == 0 ? Double.NaN : mean; }
  dbl variance() { ret count == 0 ? Double.NaN : m2/count; }
  dbl sampleVariance() { ret count < 2 ? Double.NaN : m2/(count-1); }
}