anchor.diff:    # host filesystem and thus deadlocks the system.
     networking.useDHCP = false;
   
  -  networking.defaultGateway = mkOverride 200 {} "10.0.2.2";
  +  networking.defaultGateway = mkOverride 200 "10.0.2.2";
   
     networking.nameservers = [ "10.0.2.3" ];
   
  
positive.code: 
    # When building a regular system configuration, override whatever
    # video driver the host uses.
    services.xserver.videoDriver = mkOverride 50 {} null;
    services.xserver.videoDrivers = mkOverride 50 {} [ "cirrus" "vesa" ];
    services.xserver.defaultDepth = mkOverride 50 {} 0;
    services.xserver.resolutions = mkOverride 50 {} [];
  
    services.mingetty.ttys = ttys ++ optional (!cfg.graphics) "ttyS0";
  
    # Wireless won't work in the VM.
    networking.enableWLAN = mkOverride 50 {} false;
  }
  
negative.code:   # where the regular value for the `fileSystems' attribute should be
    # disregarded for the purpose of building a VM test image (since
    # those filesystems don't exist in the VM).
    fileSystems = mkOverride 50
      [ { mountPoint = "/";
          device = "/dev/vda";
        }
  