code: func (t *Tag) Int64(i int) (int64, error) {
  	if t.format != IntVal {
  		return 0, t.typeErr(IntVal)
  	}
  	return t.intVals[i], nil
  }
docs: // Int64 returns the tag's i'th value as an integer. It returns an error if the
  // tag's Format is not IntVal. It panics if i is out of range.