code: func (t *Tag) Rat(i int) (*big.Rat, error) {
  	n, d, err := t.Rat2(i)
  	if err != nil {
  		return nil, err
  	}
  	return big.NewRat(n, d), nil
  }
docs: // Rat returns the tag's i'th value as a rational number. It returns a nil and
  // an error if this tag's Format is not RatVal.  It panics for zero deminators
  // or if i is out of range.