code: def stop(label, at=None, remove_from_labels=False, stop_once=True):
      
      t = at if at is not None else time.time()
  
      if label not in labels:
          return None
  
      timer = Marker().loads(labels[label])
  
      if timer.is_running() or (timer.is_stopped() and not stop_once):
          timer.stop(t)
  
      if remove_from_labels:
          del labels[label]
      else:
          labels[label] = timer.dumps()
  
      return timer.duration()
docs: Stops the countdown