// 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();

  }

}