anchor.diff:  
   with pkgs.lib;
   
  -let cfg = config.virtualisation.xen; in
  +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
  
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 ];
  
    };
  
  }
  