code: func (s *ReindexService) Validate() error {
  	var invalid []string
  	if s.body != nil {
  		return nil
  	}
  	if s.source == nil {
  		invalid = append(invalid, "Source")
  	} else {
  		if len(s.source.request.indices) == 0 {
  			invalid = append(invalid, "Source.Index")
  		}
  	}
  	if s.destination == nil {
  		invalid = append(invalid, "Destination")
  	}
  	if len(invalid) > 0 {
  		return fmt.Errorf("missing required fields: %v", invalid)
  	}
  	return nil
  }
docs: // Validate checks if the operation is valid.