anchor.diff:  
       system.extraSystemBuilderCmds =
         ''
  -        ln -s ${xen}/boot/xen.gz $out/xen.gz
  +        ln -s ${pkgs.xen}/boot/xen.gz $out/xen.gz
           echo "${toString cfg.bootParams}" > $out/xen-params
         '';
   
  
positive.code: 
  let
    cfg = config.virtualisation.xen;
    xen = pkgs.xen;
  in
  
  {
  
negative.code: 
  
      environment.etc =
        [ { source = "${pkgs.xen}/etc/xen/xl.conf";
            target = "xen/xl.conf";
          }
          { source = "${pkgs.xen}/etc/xen/scripts";
            target = "xen/scripts";
          }
        ];
  
      # Xen provides udev rules.
      services.udev.packages = [ pkgs.xen ];
  
      services.udev.path = [ pkgs.bridge-utils pkgs.iproute ];
  
  