code: func (tx *Tx) Indexes() ([]string, error) {
  	if tx.db == nil {
  		return nil, ErrTxClosed
  	}
  	names := make([]string, 0, len(tx.db.idxs))
  	for name := range tx.db.idxs {
  		names = append(names, name)
  	}
  	sort.Strings(names)
  	return names, nil
  }
docs: // Indexes returns a list of index names.