anchor.diff:    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
  +  # of the host into our own filesystem.  We use mkOverride 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 {}
  +  fileSystems = mkOverride 50
       [ { mountPoint = "/";
           device = "/dev/vda";
         }
  
positive.code:   # host filesystem and thus deadlocks the system.
    networking.useDHCP = false;
  
    networking.defaultGateway = mkOverrideTemplate 200 {} "10.0.2.2";
  
    networking.nameservers = [ "10.0.2.3" ];
  
  
negative.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.monitorSection =
      ''
        # Set a higher refresh rate so that resolutions > 800x600 work.
  