(define a '(1 2 (3 4) (5 6) 7 8))

(set-cdr! (car (cdr (cdr a))) (cdr a))
(begin (display a) (newline))

(set-cdr! (cdr (cdr (cdr (cdr (cdr a))))) a)
(begin (display a) (newline))

(define v (make-vector 5 '()))
(vector-set! v 2 v)
(begin (display v) (newline))

(define a '(1 2))
(define v (make-vector 2))
(set-cdr! a v)
(vector-set! v 0 a)

a
v