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:             options = [ "trans=virtio" "version=9p2000.L" ];
              neededForBoot = true;
            };
        } // optionalAttrs cfg.writableStore
        { "/nix/store" =
            { fsType = "unionfs-fuse";
              device = "unionfs";
              options = [ "allow_other" "cow" "nonempty" "chroot=/mnt-root" "max_files=32768" "hide_meta_files" "dirs=/nix/.rw-store=rw:/nix/.ro-store=ro" ];
            };
        } // optionalAttrs (cfg.writableStore && cfg.writableStoreUseTmpfs)
        { "/nix/.rw-store" =
            { fsType = "tmpfs";
  
negative.code:         description =
            ''
              If enabled, the Nix store in the VM is made writable by
              layering an overlay filesystem on top of the host's Nix
              store.
            '';
        };
  