srecord noeq StepperWithStats(Steppable steppable) > MetaWithChangeListeners is Steppable {
  settableWithVar new TimestampRange timeRange;
  settableWithVar long stepCount;
  settableWithVar bool done;
  
  public bool step() {
    if (done) false;
    if (steppable == null) false;
    
    ping();
    if (timeRange.start() == null) timeRange.startTime(tsNow());
    stepCount(stepCount+1);
    if (steppable.step()) true;
    
    // steppable ended
    timeRange.endTime(tsNow());
    done(true);
    false;
  }
  
  S renderStats() {
    S s = nSteps(stepCount);
    if (done) {
      s += " (done";
      if (timeRange.complete())
        s += " in " + formatSeconds(timeRange.duration(), 1) + "s";
      s += ")";
    }
    ret s;
  }
}