code: func (s *sendStream) handleStopSendingFrameImpl(frame *wire.StopSendingFrame) bool /*completed*/ {
  	writeErr := streamCanceledError{
  		errorCode: frame.ErrorCode,
  		error:     fmt.Errorf("Stream %d was reset with error code %d", s.streamID, frame.ErrorCode),
  	}
  	return s.cancelWriteImpl(errorCodeStopping, writeErr)
  }
docs: // must be called after locking the mutex