code: func Register(scheme string, d Backend) error {
  	if _, exists := backends[scheme]; exists {
  		return fmt.Errorf("scheme already registered %s", scheme)
  	}
  	logrus.WithField("name", scheme).Debugf("Registering discovery service")
  	backends[scheme] = d
  	return nil
  }
docs: // Register makes a discovery backend available by the provided scheme.
  // If Register is called twice with the same scheme an error is returned.