code: func (cgi *ConsumergroupInstance) UpdateRegistration(topics []string) error {
  	subscriptionJSON, err := cgi.marshalSubscription(topics)
  	if err != nil {
  		return err
  	}
  
  	node := fmt.Sprintf("%s/consumers/%s/ids/%s", cgi.cg.kz.conf.Chroot, cgi.cg.Name, cgi.ID)
  	_, stat, err := cgi.cg.kz.conn.Get(node)
  	if err != nil {
  		if err == zk.ErrNoNode {
  			return ErrInstanceNotRegistered
  		}
  		return err
  	}
  
  	_, err = cgi.cg.kz.conn.Set(node, subscriptionJSON, stat.Version)
  	if err == zk.ErrNoNode {
  		return ErrInstanceNotRegistered
  	}
  	return err
  }
docs: // UpdateRegistration updates a consumer group member registration. If the
  // consumer group member has not been registered yet, then an error is returned.