Using something like (for old-val = (atomic-decf ...)) in SBCL gives a warning that (sb-ext:truly-the ...) isn't known.

So perhaps there should be a #+sbcl (sb-ext:truly-the . walk-cddr) just below the "the" handling case.

I've reported that as https://bugs.launchpad.net/sbcl/+bug/771673, too.

Regards,

Phil