code: func (a *Attributes) True(name string) error {
  	val, ok, err := a.Value(name)
  	if err != nil {
  		return err
  	}
  	if !ok {
  		return fmt.Errorf("Attribute '%s' was not found", name)
  	}
  	if val != "true" {
  		return fmt.Errorf("Attribute '%s' is not true", name)
  	}
  	return nil
  }
docs: // True returns nil if the value of attribute 'name' is true;
  // otherwise, an appropriate error is returned.