sclass RegularlyClearedAverage > Average {
  NotTooOften clearTimer;
  
  *() {}
  *(NotTooOften *clearTimer) {}

  void add(double d) {
    possiblyClear();
    super.add(d);
  }
  
  void possiblyClear {
    if (clearTimer?!)
      clear();
  }
}