;; P07 (**) Flatten a nested list structure.
(defun my-flatten (l)
  (cond
   ((null l) nil)
   ((atom l) (list l))
   (t
    (append (my-flatten (car l))
	    (my-flatten (cdr l))))))

(my-flatten '(a (b (c d) e))) ; '(a b c d e)