docs: def cancel(self, mark_completed_as_cancelled=False):
      """
      Cancel the future. If the future has not been started yet, it will never
      start running. If the future is already running, it will run until the
      worker function exists. The worker function can check if the future has
      been cancelled using the :meth:`cancelled` method.
  
      If the future has already been completed, it will not be marked as
      cancelled unless you set *mark_completed_as_cancelled* to :const:`True`.
  
      :param mark_completed_as_cancelled: If this is :const:`True` and the
        future has already completed, it will be marked as cancelled anyway.
      """
  
      with self._lock:
        if not self._completed or mark_completed_as_cancelled:
          self._cancelled = True
        callbacks = self._prepare_done_callbacks()
      callbacks()