code: func (w *Writer) Flush() error {
  	if (!w.dirty && w.Buffered() == 0) || w.err != nil {
  		return w.err
  	}
  
  	w.err = w.flushFragment(true)
  	w.n = 0
  	w.dirty = false
  	w.fragmented = false
  
  	return w.err
  }
docs: // Flush writes any buffered data to the underlying io.Writer.
  // It sends the frame with "fin" flag set to true.
  //
  // If no Write() or ReadFrom() was made, then Flush() does nothing.