I had a brief go at this myself, but I quickly got stuck.

  * mmdebstrap always has tar write to stdout, rather than adding "-f 
$filename" to @taropts.

    mksquashfs doesn't allow this.
    mksquashfs *has to* write to a regular file (AFAIK).

  * mmdebstrap adds compression to the pipeline itself, rather than adding e.g. 
"--zstd" to @taropts.

    mksquashfs doesn't allow this.
    mksquashfs *has to* do compression itself.

  * mmdebstrap avoids avoid mknod() by assembling a "/dev only" uncompressed 
tar from raw bytes ($devtar),
    and just printing it to stdout before the main tar run.  i.e. it relies on 
tape archives being concatenative.

    mksquashfs doesn't allow this.
    mksquashfs *can* append to an existing tarball, but (again) they have to be 
regular files.

    So mmdebstrap would have to do something like

        print $devsq >unstable-chroot.squashfs
        mksquashfs root $filename

    This also has implications for reproducible builds when the mode changes (I 

  * Note that appending to an existing file is the *default behaviour*.
    To avoid this, you need -noappend.
    This can trick you if you do multiple builds to the same destination, e.g.

      mksquashfs attempt-1/ final.squashfs
      mksquashfs attempt-2/ final.squashfs   # weird errors, because it 
overwrite the old final.squashfs contents!

I did also consider piping, something like:

    mmdebstrap unstable | tar2squashfs unstable.squashfs

but such a command doesn't exist, and would preclude some of
squashfs's compression and reproducible builds functionality.

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