code: func (w *Writer) FlushFragment() error {
  	if w.Buffered() == 0 || w.err != nil {
  		return w.err
  	}
  
  	w.err = w.flushFragment(false)
  	w.n = 0
  	w.fragmented = true
  
  	return w.err
  }
docs: // FlushFragment writes any buffered data to the underlying io.Writer.
  // It sends the frame with "fin" flag set to false.