# File lib/spreadsheet/excel/workbook.rb, line 51
  def biff_version
    case @bof
    when 0x009
      2
    when 0x209
      3
    when 0x409
      4
    else
      BIFF_VERSIONS.fetch(@version) { raise "Unkown BIFF_VERSION '#@version'" }
    end
  end