def create_or_open_sheet_at index
      s = self[index]
      s = self[index] = Workbook::Sheet.new if s == nil
      s.book = self
      s
    end
def create_or_open_sheet_at index
      s = self[index]
      s = self[index] = Workbook::Sheet.new if s == nil
      s.book = self
      s
    end
Create or open the existing sheet at an index value

 @param [Integer] index    the index of the sheet