code: func Set(featureSet map[string]bool) error {
  	fMu.Lock()
  	defer fMu.Unlock()
  	for n, v := range featureSet {
  		f, present := nameToFeature[n]
  		if !present {
  			return fmt.Errorf("feature '%s' doesn't exist", n)
  		}
  		features[f] = v
  	}
  	return nil
  }
docs: // Set accepts a list of features and whether they should
  // be enabled or disabled, it will return a error if passed
  // a feature name that it doesn't know