anchor.diff:  
     config = mkIf cfg.enable {
   
  -    environment.systemPackages = [ pkgs.xen ];
  +    environment.systemPackages = [ xen ];
   
       # Domain 0 requires a pvops-enabled kernel.
       boot.kernelPackages = pkgs.linuxPackages_2_6_32_xen;
   
  -    boot.kernelModules = [ "xen_evtchn" "xen_gntdev" ];
  +    boot.kernelModules = [ "xen_evtchn" "xen_gntdev" "xen_blkback" "xen_netback" "xen_pciback" "blktap" ];
   
       # The radeonfb kernel module causes the screen to go black as soon
       # as it's loaded, so don't load it.
  
positive.code: 
      system.extraSystemBuilderCmds =
        ''
          ln -s ${pkgs.xen}/boot/xen.gz $out/xen.gz
          echo "${toString cfg.bootParams}" > $out/xen-params
        '';
  
  
negative.code: 
  with pkgs.lib;
  
  let 
  
    cfg = config.virtualisation.xen; 
  
    xen = pkgs.xen;
  
    xendConfig = pkgs.writeText "xend-config.sxp"
      ''
        (loglevel DEBUG)
        (network-script network-bridge)
        (vif-script vif-bridge)
      '';
  
  in
  
  {
    ###### interface
  