Source code in the common programming language

(defun flatten (structure)
  (cond ((null structure) nil)
        ((atom structure) (list structure))
        (t (mapcan #'flatten structure))))


(defun flatten (ls)
  (labels ((mklist (x) (if (listp x) x (list x))))
    (mapcan #'(lambda (x) (if (atom x) (mklist x) (flatten x))) ls)))


(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)))


(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)))


(defun flatten (obj)
  (do* ((result (list obj))
        (node result))
       ((null node) (delete nil result))
    (cond ((consp (car node))
           (when (cdar node) (push (cdar node) (cdr node)))
           (setf (car node) (caar node)))
          (t (setf node (cdr node))))))