anchor.diff:      client = 
         {config, pkgs, ...}:
         {
  -        services.xserver.enable = true;
  -        services.xserver.displayManager.slim.enable = false;
  -        services.xserver.displayManager.kdm.enable = true;
  -        services.xserver.displayManager.kdm.extraConfig =
  -          ''
  -            [X-:0-Core]
  -            AutoLoginEnable=true
  -            AutoLoginUser=alice
  -            AutoLoginPass=foobar
  -          '';
  -        services.xserver.desktopManager.default = "kde4";
  +        require = [ ./common/x11.nix ];
           services.xserver.desktopManager.kde4.enable = true;
  -        users.extraUsers = pkgs.lib.singleton { 
  -          name = "alice";
  -          description = "Alice Foobar";
  -          home = "/home/alice";
  -          createHome = true;
  -          useDefaultShell = true;
  -          password = "foobar";
  -        };
         };
     };
       
  
positive.code:       $webserver->mustSucceed("PYTHONPATH=${pkgs.pythonPackages.psycopg2}/lib/python2.5/site-packages trac-admin /var/trac/projects/test initenv Test postgres://root\@postgresql/trac svn /repos/trac");
        
        $client->waitForFile("/tmp/.X11-unix/X0");
        sleep 60;
        
        $client->execute("su - alice -c 'DISPLAY=:0.0 konqueror http://webserver/projects/test &'");
        sleep 120;
        
        $client->screenshot("screen");
      '';
  
negative.code:       {config, pkgs, ...}:
        {
          fileSystems = pkgs.lib.mkOverride 50 {} 
            [ { mountPoint = "/repos";
                device = "storage:/repos"; } 
            ];
        
          services.portmap.enable = true;
          services.httpd.enable = true;
  