def to_s(format = :default, separator = ': ')
      case format
      when :label
        return [label, number].compact.join(separator)
      else
        return number
      end
    end
def to_s(format = :default, separator = ': ')
      case format
      when :label
        return [label, number].compact.join(separator)
      else
        return number
      end
    end
String