(DEFUN COMP (N &OPTIONAL REZ)
    (COND
        ((< N 10) (CONS N REZ))
        (T (COMP
            (/ (- N (rem N 10)) 10)
            (CONS (rem N 10) REZ)
        ))
    )
)

(write (COMP 5678394)) ; (5 6 7 8 3 9 4)