anchor.diff:        '';
   
       boot.initrd.availableKernelModules =
  -      optional (cfg.qemu.diskInterface == "scsi") "sym53c8xx";
  +      optional cfg.writableStore "overlay"
  +      ++ optional (cfg.qemu.diskInterface == "scsi") "sym53c8xx";
   
       virtualisation.bootDevice =
         mkDefault (if cfg.qemu.diskInterface == "scsi" then "/dev/sda" else "/dev/vda");
  
positive.code:         description =
            ''
              If enabled, the Nix store in the VM is made writable by
              layering a unionfs-fuse/tmpfs filesystem on top of the host's Nix
              store.
            '';
        };
  
negative.code:         chmod 1777 $targetRoot/tmp
  
          mkdir -p $targetRoot/boot
  
          ${optionalString cfg.writableStore ''
            echo "mounting overlay filesystem on /nix/store..."
            mkdir -p 0755 $targetRoot/nix/.rw-store/store $targetRoot/nix/.rw-store/work $targetRoot/nix/store
            mount -t overlay overlay $targetRoot/nix/store \
              -o lowerdir=$targetRoot/nix/.ro-store,upperdir=$targetRoot/nix/.rw-store/store,workdir=$targetRoot/nix/.rw-store/work || fail
          ''}
        '';
  
      # After booting, register the closure of the paths in
  