code: func (cs *clientStore) setClientHB(ID string, interval time.Duration, f func()) {
  	cs.RLock()
  	defer cs.RUnlock()
  	c := cs.clients[ID]
  	if c == nil {
  		return
  	}
  	c.Lock()
  	if c.hbt == nil {
  		c.hbt = time.AfterFunc(interval, f)
  	}
  	c.Unlock()
  }
docs: // setClientHB will lookup the client `ID` and, if present, set the
  // client's timer with the given interval and function.