code: func Enabled(n FeatureFlag) bool {
  	fMu.RLock()
  	defer fMu.RUnlock()
  	v, present := features[n]
  	if !present {
  		panic(fmt.Sprintf("feature '%s' doesn't exist", n.String()))
  	}
  	return v
  }
docs: // Enabled returns true if the feature is enabled or false
  // if it isn't, it will panic if passed a feature that it
  // doesn't know.