[plt-scheme] mutable lists/dicts?

Hi,

Experimenting for the first time with mutable lists (I want a mutable assoc list):

> (dict? (list (list 1 2) (list 12)))
#t

> (dict-mutable? (list (list 1 2) (list 1 2)))
#f

--> ok, so let's move to mutable lists!

> (dict? (mlist (mlist 1 2) (mlist 1 2)))
#f

> (dict-mutable? (mlist (mlist 1 2) (mlist 2 3)))
dict-mutable?: expected argument of type <dict>; given {{1 2} {2 3}}

--> what??

I must be missing something! any idea?

Thanks,

-- Éric