code: func (s *ReindexService) DestinationIndex(index string) *ReindexService {
  	if s.destination == nil {
  		s.destination = NewReindexDestination()
  	}
  	s.destination = s.destination.Index(index)
  	return s
  }
docs: // DestinationIndex specifies the destination index of the reindexing process.