code: func (f *DeltaFIFO) addIfNotPresent(id string, deltas Deltas) {
  	f.populated = true
  	if _, exists := f.items[id]; exists {
  		return
  	}
  
  	f.queue = append(f.queue, id)
  	f.items[id] = deltas
  	f.cond.Broadcast()
  }
docs: // addIfNotPresent inserts deltas under id if it does not exist, and assumes the caller
  // already holds the fifo lock.