code: func (f *DeltaFIFO) KeyOf(obj interface{}) (string, error) {
  	if d, ok := obj.(Deltas); ok {
  		if len(d) == 0 {
  			return "", KeyError{obj, ErrZeroLengthDeltasObject}
  		}
  		obj = d.Newest().Object
  	}
  	if d, ok := obj.(DeletedFinalStateUnknown); ok {
  		return d.Key, nil
  	}
  	return f.keyFunc(obj)
  }
docs: // KeyOf exposes f's keyFunc, but also detects the key of a Deltas object or
  // DeletedFinalStateUnknown objects.