[plt-scheme] mzscheme behaviour differs between debian and gentoo

Hi,

the script:

> (define a (list 1 2))
> (define b (list 3 4))
> (append! a b)
(1 2 3 4)
> a
(1 2)
> b
(3 4)

provides not the expected result (here running unter gentoo)

unter debian the output of 

> a

would be

(1 2 3 4)

what could the problem be?

regards.