code: func (f *FIFO) addIfNotPresent(id string, obj interface{}) {
  	f.populated = true
  	if _, exists := f.items[id]; exists {
  		return
  	}
  
  	f.queue = append(f.queue, id)
  	f.items[id] = obj
  	f.cond.Broadcast()
  }
docs: // addIfNotPresent assumes the fifo lock is already held and adds the provided
  // item to the queue under id if it does not already exist.