code: func (rww *responseWriterWrapper) Flush() {
  	if f, ok := rww.w.(http.Flusher); ok {
  		f.Flush()
  	} else {
  		panic(httpserver.NonFlusherError{Underlying: rww.w}) // should be recovered at the beginning of middleware stack
  	}
  }
docs: // Flush implements http.Flusher. It simply wraps the underlying
  // ResponseWriter's Flush method if there is one, or panics.