[plt-scheme] Mouse wheel

Hi,

I have a graphical program in PLT-scheme. In the program
there are
mouse and keyboard actions. I have tried to write the
program in such
a way, that the GUI is separated from the underlying mechanism.

Now I have a problem with the mouse wheel, since in
PLT-scheme it is
a keyboard action. Is there a way to "redirect" the mouse
wheel actions
so they will appear in the "on-event" event of a canvas%???

I do not know whether it could be a solution but it caught
my eye in the
documentation where it says, that a keymap% can be used not
only with editors but with other classes as well. How can I
do that? Can I use it in a canvas%? 

Maybe I misunderstood something.

Thanks for any help,

Peter Ivanyi