code: func (s *ScrollService) Preference(preference string) *ScrollService {
  	s.preference = preference
  	return s
  }
docs: // Preference sets the preference to execute the search. Defaults to
  // randomize across shards ("random"). Can be set to "_local" to prefer
  // local shards, "_primary" to execute on primary shards only,
  // or a custom value which guarantees that the same order will be used
  // across different requests.