[plt-scheme] append!?

Matthias Felleisen wrote:
> I don't know what functional programming is but Kazimir's type 
> description is perfectly fine data description for Scheme:
>
>   ListB is (box List)
>
...
> All he loses is the immediate applicability of the existing library. 
> Ugly but doable.
>
> Unless he meant something else. -- Matthias
Yes, that's exactly what I meant, including the loss. :)