RosettaCodeData/Task/Flatten-a-list/Common-Lisp/flatten-a-list-3.lisp

(defun flatten (obj)
  (let (result)
    (labels ((grep (obj)
               (cond ((null obj) nil)
                     ((atom obj) (push obj result))
                     (t (grep (rest obj))
                        (grep (first obj))))))
      (grep obj)
      result)))