anchor.diff:    # 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 {}
  +  fileSystems = mkOverride 50
       [ { mountPoint = "/";
           device = "/dev/vda";
         }
  
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:   # host filesystem and thus deadlocks the system.
    networking.useDHCP = false;
  
    networking.defaultGateway = mkOverride 200 "10.0.2.2";
  
    networking.nameservers = [ "10.0.2.3" ];
  
  