code: func (result ObserveResult) CheckPersistence(cas uint64, deletion bool) (persisted bool, overwritten bool) {
  	switch {
  	case result.Status == ObservedNotFound && deletion:
  		persisted = true
  	case result.Cas != cas:
  		overwritten = true
  	case result.Status == ObservedPersisted:
  		persisted = true
  	}
  	return
  }
docs: // CheckPersistence checks whether a stored value has been persisted to disk yet.