code: func (f *DeltaFIFO) Add(obj interface{}) error {
  	f.lock.Lock()
  	defer f.lock.Unlock()
  	f.populated = true
  	return f.queueActionLocked(Added, obj)
  }
docs: // Add inserts an item, and puts it in the queue. The item is only enqueued
  // if it doesn't already exist in the set.