;; With mapcan
(defun my-flatten (lst)
  (mapcan #'(lambda (item)
	      (if (listp item)
		  (my-flatten item)
		  (list item))
	      ) lst))

;; With reduce
(defun my-flatten (lst)
  (reduce #'(lambda (accumulator item)
	      (if (listp item)
		  (append accumulator (my-flatten item))
		  (append accumulator (list item)))
	      ) lst :initial-value '()))


;; C-c C-k
(assert (equal (my-flatten '()) '()))
(assert (equal (my-flatten '(a b c)) '(A B C)))
(assert (equal (my-flatten '((a b) c d)) '(A B C D)))
(assert (equal (my-flatten '(a (b (c d) e))) '(A B C D E)))