code: func (t *Tag) StringVal() (string, error) {
  	if t.format != StringVal {
  		return "", t.typeErr(StringVal)
  	}
  	return t.strVal, nil
  }
docs: // StringVal returns the tag's value as a string. It returns an error if the
  // tag's Format is not StringVal. It panics if i is out of range.