> (define (flatten x)
    (cond ((null? x) '())
          ((not (pair? x)) (list x))
          (else (append (flatten (car x))
                        (flatten (cdr x))))))

> (flatten '((1) 2 ((3 4) 5) ((())) (((6))) 7 8 ()))
(1 2 3 4 5 6 7 8)