flatten.el

(setq *xs* '(1 (2 (3 (4 5 6) 7) 8 9 ) 10 11))

(defun flatten (xs)
  (unless (null xs)
    (let ((head (car xs))
          (tail (cdr xs)))
      ;; (message "head=%s tail=%s" (car xs) (cdr xs))
      (if (atom head)
          (cons head (flatten tail))
        (append (flatten head) (flatten tail))))))

(flatten *xs*)
(1 2 3 4 5 6 7 8 9 10 11)