code: func ModifyConfig(c context.Context, cb func(Config) Config) context.Context {
  	var cfg Config
  	if cur := getConfig(c); cur != nil {
  		cfg = *cur
  	}
  	cfg = cb(cfg)
  	return setConfig(c, &cfg)
  }
docs: // ModifyConfig makes a context with a derived configuration.
  //
  // It grabs current configuration from the context (if any), passes it to the
  // callback, and puts whatever callback returns into a derived context.