srecord noeq StepperWithStats(Steppable steppable) > MetaWithChangeListeners is Steppable {
  settableWithVar Timestamp startTime;
  settableWithVar Timestamp endTime;
  settableWithVar long stepCount;
  settableWithVar bool done;
  
  public bool step() {
    if (done) false;
    if (steppable == null) false;
    
    ping();
    if (startTime == null) startTime(tsNow());
    stepCount(stepCount+1);
    if (steppable.step()) true;
    
    // steppable ended
    endTime(tsNow());
    done(true);
    false;
  }
  
  S renderStats() {
    ret nSteps(stepCount);
  }
}