code: func (f *DeltaFIFO) GetByKey(key string) (item interface{}, exists bool, err error) {
  	f.lock.RLock()
  	defer f.lock.RUnlock()
  	d, exists := f.items[key]
  	if exists {
  		// Copy item's slice so operations on this slice (delta
  		// compression) won't interfere with the object we return.
  		d = copyDeltas(d)
  	}
  	return d, exists, nil
  }
docs: // GetByKey returns the complete list of deltas for the requested item,
  // setting exists=false if that list is empty.
  // You should treat the items returned inside the deltas as immutable.