code: func (t *PCPTimer) Start() error {
  	t.mutex.Lock()
  	defer t.mutex.Unlock()
  
  	if t.started {
  		return errors.New("trying to start an already started timer")
  	}
  
  	t.since = time.Now()
  	t.started = true
  	return nil
  }
docs: // Start signals the timer to start monitoring.