anchor.diff:  
     # When building a regular system configuration, override whatever
     # video driver the host uses.
  -  services.xserver.videoDriver = mkOverrideTemplate 50 {} null;
  -  services.xserver.videoDrivers = mkOverrideTemplate 50 {} [ "cirrus" "vesa" ];
  -  services.xserver.defaultDepth = mkOverrideTemplate 50 {} 0;
  +  services.xserver.videoDriver = mkOverride 50 null;
  +  services.xserver.videoDrivers = mkOverride 50 [ "cirrus" "vesa" ];
  +  services.xserver.defaultDepth = mkOverride 50 0;
     services.xserver.monitorSection =
       ''
         # Set a higher refresh rate so that resolutions > 800x600 work.
  
positive.code:   virtualisation.pathsInNixDB = [ config.system.build.toplevel ];
    
    # Mount the host filesystem via CIFS, and bind-mount the Nix store
    # of the host into our own filesystem.  We use mkOverrideTemplate to allow
    # this module to be applied to "normal" NixOS system configuration,
    # 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 = mkOverrideTemplate 50 {}
      [ { mountPoint = "/";
          device = "/dev/vda";
        }
  
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" ];
  
  