code: func (s *ScrollService) bodyNext() (interface{}, error) {
  	s.mu.RLock()
  	body := struct {
  		Scroll   string `json:"scroll"`
  		ScrollId string `json:"scroll_id,omitempty"`
  	}{
  		Scroll:   s.keepAlive,
  		ScrollId: s.scrollId,
  	}
  	s.mu.RUnlock()
  	return body, nil
  }
docs: // body returns the request to fetch the next batch of results.