anchor.diff:          default = true;
           description =
             ''
  -            Whether to run qemu with a graphics window, or access
  +            Whether to run QEMU with a graphics window, or access
               the guest computer serial port through the host tty.
             '';
         };
   
  +    virtualisation.pathsInNixDB =
  +      mkOption {
  +        default = [];
  +        description =
  +          ''
  +            The list of paths whose closure is registered in the Nix
  +            database in the VM.  All other paths in the host Nix store
  +            appear in the guest Nix store as well, but are considered
  +            garbage (because they are not registered in the Nix
  +            database in the guest).
  +          '';
  +      };
  +      
     };
   
     cfg = config.virtualisation;
  
positive.code:           -initrd ${config.system.build.toplevel}/initrd \
            ${qemuGraphics} \
            $QEMU_OPTS \
            -append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.bootStage2} systemConfig=${config.system.build.toplevel} ${kernelConsole} $QEMU_KERNEL_PARAMS"
      '';
  
  in
  
negative.code:       fi
      '';
  
    # After booting, register the closure of the paths in
    # `virtualisation.pathsInNixDB' in the Nix database in the VM.  This
    # allows Nix operations to work in the VM.  The path to the
    # registration file is passed through the kernel command line to
    # allow `system.build.toplevel' to be included.  (If we had a direct
    # reference to ${regInfo} here, then we would get a cyclic
    # dependency.)
    boot.postBootCommands =
      ''
        ( source /proc/cmdline
          ${config.environment.nix}/bin/nix-store --load-db < $regInfo
        )
      '';
        
    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 mkOverride to allow
    # this module to be applied to "normal" NixOS system configuration,
  