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