anchor.diff:  # the VM in the host.  On the other hand, the root filesystem is a
   # read/writable disk image persistent across VM reboots.
   
  -{ config, lib, pkgs, ... }:
  +{ config, lib, pkgs, options, ... }:
   
   with lib;
   with import ../../lib/qemu-flags.nix { inherit pkgs; };
  
positive.code:     # attribute should be disregarded for the purpose of building a VM
      # test image (since those filesystems don't exist in the VM).
      fileSystems = mkVMOverride (
        { "/".device = cfg.bootDevice;
          ${if cfg.writableStore then "/nix/.ro-store" else "/nix/store"} =
            { device = "store";
  
negative.code: 
    options = {
  
      virtualisation.fileSystems = options.fileSystems;
  
      virtualisation.memorySize =
        mkOption {
          default = 384;
  