def os
  @os ||= (
    host_os = RbConfig::CONFIG['host_os']
    case host_os
      when /mswin|msys|mingw|cygwin|bccwin|wince|emc/
        :windows
      when /darwin|mac os/
        :macosx
      when /linux/
        :linux
      when /solaris|bsd/
        :unix
      else
        raise Error::WebDriverError, "unknown os: #{host_os.inspect}"
    end
  )
end
