// from https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm
srecord HalfWelford(long count, dbl mean) {
  void add(double newValue) {
    count++;
    dbl delta = newValue - mean;
    mean += delta / count;
  }
  
  void remove(double valueToRemove) {
    guarantee(count > 0);
    
    double newValue = valueToRemove;
    mean = (mean-newValue/count)/(1-1.0/count);
    --count;
  }
 
  bool isEmpty() { ret count == 0; }

  dbl average aka avg aka mean() { ret count == 0 ? Double.NaN : mean; }

  toString {
    S s = nValues(count);
    if (!isEmpty())
      s += ", avg: " + avg();
    ret s;
  }
  
  double get() {
    ret avg();
  }
}

Began life as a copy of #1033575