anchor.diff:  
     # Always include cryptsetup so that Charon can use it.
     environment.systemPackages = [ pkgs.cryptsetup ];
  +
  +  boot.initrd.supportedFilesystems = [ "unionfs-fuse" ];
   }
  
positive.code:   # while "m1.large" has two ephemeral filesystems and no swap
    # devices).  Also, put /tmp and /var on /disk0, since it has a lot
    # more space than the root device.  Similarly, "move" /nix to /disk0
    # by layering an AUFS on top of it so we have a lot more space for
    # Nix operations.
    boot.initrd.postMountCommands =
      ''
        diskNr=0
        diskForAufs=
        for device in /dev/xvd[abcde]*; do
            if [ "$device" = /dev/xvda -o "$device" = /dev/xvda1 ]; then continue; fi
            fsType=$(blkid -o value -s TYPE "$device" || true)
  
negative.code:               diskNr=$((diskNr + 1))
                echo "mounting $device on $mp..."
                if mountFS "$device" "$mp" "" ext3; then
                    if [ -z "$diskForUnionfs" ]; then diskForUnionfs="$mp"; fi
                fi
            else
                echo "skipping unknown device type $device"
            fi
        done
  
        if [ -n "$diskForUnionfs" ]; then
            mkdir -m 755 -p $targetRoot/$diskForUnionfs/root
  
            mkdir -m 1777 -p $targetRoot/$diskForUnionfs/root/tmp $targetRoot/tmp
            mount --bind $targetRoot/$diskForUnionfs/root/tmp $targetRoot/tmp
  
            if [ ! -e $targetRoot/.ebs ]; then
                mkdir -m 755 -p $targetRoot/$diskForUnionfs/root/var $targetRoot/var
                mount --bind $targetRoot/$diskForUnionfs/root/var $targetRoot/var
  
                mkdir -p /unionfs-chroot/ro-nix
                mount --rbind $targetRoot/nix /unionfs-chroot/ro-nix
  
                mkdir -m 755 -p $targetRoot/$diskForUnionfs/root/nix
                mkdir -p /unionfs-chroot/rw-nix
                mount --rbind $targetRoot/$diskForUnionfs/root/nix /unionfs-chroot/rw-nix
  
                unionfs -o allow_other,cow,nonempty,chroot=/unionfs-chroot /rw-nix=RW:/ro-nix=RO $targetRoot/nix
            fi
        fi
      '';
  