def validate_workbook()
    unless @workbook.nil? || @workbook.worksheets.nil?
      return if @workbook.worksheets.any? { |sheet| sheet.equal?(self) }
    end

    raise "This worksheet #{self} is not in workbook #{@workbook}"
  end
def validate_workbook()
    unless @workbook.nil? || @workbook.worksheets.nil?
      return if @workbook.worksheets.any? { |sheet| sheet.equal?(self) }
    end

    raise "This worksheet #{self} is not in workbook #{@workbook}"
  end
validates Workbook, ensures that this worksheet is in @workbook