code: func (s *IndicesForcemergeService) Index(index ...string) *IndicesForcemergeService {
  	if s.index == nil {
  		s.index = make([]string, 0)
  	}
  	s.index = append(s.index, index...)
  	return s
  }
docs: // Index is a list of index names; use `_all` or empty string to perform
  // the operation on all indices.