RosettaCodeData/Task/Flatten-a-list/Common-Lisp/flatten-a-list-4.lisp

(defun flatten (x &optional stack out)
  (cond ((consp x) (flatten (rest x) (cons (first x) stack) out))
        (x         (flatten (first stack) (rest stack) (cons x out)))
        (stack     (flatten (first stack) (rest stack) out))
        (t out)))