code: func (tx *Tx) GetLess(index string) (func(a, b string) bool, error) {
  	if tx.db == nil {
  		return nil, ErrTxClosed
  	}
  	idx, ok := tx.db.idxs[index]
  	if !ok || idx.less == nil {
  		return nil, ErrNotFound
  	}
  	return idx.less, nil
  }
docs: // GetLess returns the less function for an index. This is handy for
  // doing ad-hoc compares inside a transaction.
  // Returns ErrNotFound if the index is not found or there is no less
  // function bound to the index