code: func CachableResponseWriter(req *http.Request,
  	statusCode int,
  	resp http.ResponseWriter,
  	opts Options) ([]cacheobject.Reason, time.Time, error) {
  	return cacheobject.UsingRequestResponse(req, statusCode, resp.Header(), opts.PrivateCache)
  }
docs: // Given an HTTP Request, the future Status Code, and an ResponseWriter,
  // determine the possible reasons a response SHOULD NOT be cached.