def [](ind)
      case ind
      when Integer then worksheets[ind]
      when String  then worksheets.find { |ws| ws.sheet_name == ind }
      end
    end
def [](ind)
      case ind
      when Integer then worksheets[ind]
      when String  then worksheets.find { |ws| ws.sheet_name == ind }
      end
    end
Finds worksheet by its name or numerical index