code: func (e Enum) GetValue(key string) (interface{}, error) {
  	for k, v := range e {
  		if k == key {
  			return v, nil
  		}
  	}
  	return nil, fmt.Errorf("flagenum: Invalid value; must be one of [%s]", e.Choices())
  }
docs: // GetValue returns the mapped enumeration value associated with a key.