x (10 20 30 4 5) (rplacd x '(2 3 4)) (2 3 4) x (10 2 3 4) (rplacd (cddr x) '(30 40 50)) (30 40 50) x (10 2 3 30 40 50)
(setq x '(1 2 3 4)) (1 2 3 4) (rplacd x (cddr x)) (3 4) x (1 3 4)