code: func (i *Index) openExistenceField() error {
  	f, err := i.createFieldIfNotExists(existenceFieldName, FieldOptions{CacheType: CacheTypeNone, CacheSize: 0})
  	if err != nil {
  		return errors.Wrap(err, "creating existence field")
  	}
  	i.existenceFld = f
  	return nil
  }
docs: // openExistenceField gets or creates the existence field and associates it to the index.