docs: def stop(self):
          """Stop a task immediately.
  
          Raises:
              RuntimeError: If the task hasn't been started or has already been
                  stopped.
          """
          if self._status is TaskStatus.STOPPED:
              return
  
          if self._status is not TaskStatus.STARTED:
              raise RuntimeError("Cannot stop %s in state %s" %
                                 (self, self._status))
          self._stop()
  
          STARTED_TASKS.remove(self)
          self._status = TaskStatus.STOPPED