anchor.diff:      ] ++ optional cfg.useBootLoader
         { mountPoint = "/boot";
           device = "/dev/disk/by-label/boot";
  -        fsType = "ext3";
  +        fsType = "ext4";
           options = "ro";
           noCheck = true; # fsck fails on a r/o filesystem
         });
  
positive.code:       # initialise.
        FSTYPE=$(blkid -o value -s TYPE /dev/vda || true)
        if test -z "$FSTYPE"; then
            mke2fs -t ext3 /dev/vda
        fi
      '';
  
  
negative.code:   # Wireless won't work in the VM.
    networking.wireless.enable = mkOverride 50 false;
  
    system.requiredKernelConfig = with config.lib.kernelConfig;
      [ (isEnabled "VIRTIO_BLK")
        (isEnabled "VIRTIO_PCI")
        (isEnabled "VIRTIO_NET")
        (isEnabled "EXT4_FS")
        (isEnabled "CIFS")
        (isYes "BLK_DEV")
        (isYes "PCI")
        (isYes "EXPERIMENTAL")
        (isYes "NETDEVICES")
        (isYes "NET_CORE")
        (isYes "INET")
        (isYes "NETWORK_FILESYSTEMS")
      ] ++ optional (!cfg.graphics) [
        (isYes "SERIAL_8250_CONSOLE")
        (isYes "SERIAL_8250")
      ];
  }
  