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