anchor.diff:  
       system.extraSystemBuilderCmds =
         ''
  -        ln -s ${pkgs.xen}/boot/xen.gz $out/xen.gz
  +        ln -s ${xen}/boot/xen.gz $out/xen.gz
           echo "${toString cfg.bootParams}" > $out/xen-params
         '';
   
  
positive.code: 
          path = 
            [ pkgs.bridge_utils pkgs.gawk pkgs.iproute pkgs.nettools 
              pkgs.utillinux pkgs.bash pkgs.xen pkgs.pciutils pkgs.procps
            ];
  
          preStart = "${pkgs.xen}/sbin/xend start";
  
          postStop = "${pkgs.xen}/sbin/xend stop";
        };
  
      # To prevent a race between dhclient and xend's bridge setup
  
negative.code:     # virtual device), start dhclient after xend.
      jobs.dhclient.startOn = mkOverride 50 "started xend";
  
      environment.etc =
        [ { source = xendConfig;
            target = "xen/xend-config.sxp";
          }
          { source = "${xen}/etc/xen/scripts";
            target = "xen/scripts";
          }
        ];
  
      # Xen provides udev rules.
      services.udev.packages = [ xen ];
  
      services.udev.path = [ pkgs.bridge_utils pkgs.iproute ];
  
    };
  
  }
  