code: func (t *Tag) Rat2(i int) (num, den int64, err error) {
  	if t.format != RatVal {
  		return 0, 0, t.typeErr(RatVal)
  	}
  	return t.ratVals[i][0], t.ratVals[i][1], nil
  }
docs: // Rat2 returns the tag's i'th value as a rational number represented by a
  // numerator-denominator pair. It returns an error if the tag's Format is not
  // RatVal. It panics if i is out of range.