def to_s
      @to_s ||= begin
        wks = @weeks.map { |w|
          w.map { |d|
            if d.nil?
              '  '
            elsif d.day < 10
              " #{d.day}"
            else
              "#{d.day}"
            end
          }.join(' ')
        }.join("\n")
        "Su Mo Tu We Th Fr Sa\n#{wks}\n"
      end
    end
def to_s
      @to_s ||= begin
        wks = @weeks.map { |w|
          w.map { |d|
            if d.nil?
              '  '
            elsif d.day < 10
              " #{d.day}"
            else
              "#{d.day}"
            end
          }.join(' ')
        }.join("\n")
        "Su Mo Tu We Th Fr Sa\n#{wks}\n"
      end
    end
Initializes an instance from a year and a month. Raises an error if the
 month is not in the allowed range, i.e. it must be between 1 and 12 inclusive.