code: func (cs *clientStore) addSub(ID string, sub *subState) bool {
  	cs.RLock()
  	defer cs.RUnlock()
  	c := cs.clients[ID]
  	if c == nil {
  		return false
  	}
  	c.Lock()
  	c.subs = append(c.subs, sub)
  	c.Unlock()
  	return true
  }
docs: // AddSub adds the subscription to the client identified by clientID
  // and returns true only if the client has not been unregistered,
  // otherwise returns false.