code: func (c *Controller) Unsubscribe(name string) error {
  	c.mu.Lock()
  	defer c.mu.Unlock()
  	sub, ok := c.subscriptions[name]
  	if !ok {
  		return fmt.Errorf("Unknown subscription '%s'", name)
  	}
  	msg := NewMsg(MsgCodeStop, name, sub.address)
  	smsg, err := rlp.EncodeToBytes(msg)
  	if err != nil {
  		return err
  	}
  	err = c.pss.SendAsym(sub.pubkeyId, controlTopic, smsg)
  	if err != nil {
  		return err
  	}
  	delete(c.subscriptions, name)
  	return nil
  }
docs: // Unsubscribe, perhaps unsurprisingly, undoes the effects of Subscribe
  // Fails if the subscription does not exist, if the request pss cannot be sent or if the update message could not be serialized