code: func OneOf(choices []string) Func {
  	return func(input string) error {
  		for _, v := range choices {
  			if input == v {
  				return nil
  			}
  		}
  
  		return NewValidationError(fmt.Sprintf("%s is not a valid option.", input))
  	}
  }
docs: // OneOf returns a validation function which validates whether or not the input
  // matches one of the given options.