anchor.diff:            };
         } // optionalAttrs cfg.useBootLoader
         { "/boot" =
  -          { device = "/dev/vdb2";
  +          { device = "${driveDeviceName 1}2";
               fsType = "vfat";
               options = [ "ro" ];
               noCheck = true; # fsck fails on a r/o filesystem
  
positive.code:       optional cfg.writableStore "overlay"
        ++ optional (cfg.qemu.diskInterface == "scsi") "sym53c8xx";
  
      virtualisation.bootDevice =
        mkDefault (if cfg.qemu.diskInterface == "scsi" then "/dev/sda" else "/dev/vda");
  
      virtualisation.pathsInNixDB = [ config.system.build.toplevel ];
  
  
negative.code:       "-drive ${driveOpts} ${device}";
  
    drivesCmdLine = drives: concatStringsSep " " (imap1 driveCmdline drives);
  
    # Creates a device name from a 1-based a numerical index, e.g.
    # * `driveDeviceName 1` -> `/dev/vda`
    # * `driveDeviceName 2` -> `/dev/vdb`
    driveDeviceName = idx:
      let letter = elemAt lowerChars (idx - 1);
      in if cfg.qemu.diskInterface == "scsi" then
        "/dev/sd${letter}"
      else
        "/dev/vd${letter}";
  
    # Shell script to start the VM.
    startVM =
  