code: func (tx *Tx) GetRect(index string) (func(s string) (min, max []float64),
  	error) {
  	if tx.db == nil {
  		return nil, ErrTxClosed
  	}
  	idx, ok := tx.db.idxs[index]
  	if !ok || idx.rect == nil {
  		return nil, ErrNotFound
  	}
  	return idx.rect, nil
  }
docs: // GetRect returns the rect function for an index. This is handy for
  // doing ad-hoc searches inside a transaction.
  // Returns ErrNotFound if the index is not found or there is no rect
  // function bound to the index