[plt-scheme] extending gui.ss

I recently had a question from a teacher in Germany about my extensions 
to the gui.ss teachpack, which made me realize that I don't understand 
gui.ss as well as I should.

Callbacks associated with MrEd objects are provided with the object and 
the event which triggered the callback; gui.ss provides just the event 
to the user-supplied callback function. HtDP readers don't know what to 
do with this and have to ignore it. I thought it might make sense, for a 
button, to provide the button to the user-supplied callback function. 
Then one function could handle multiple buttons.

But a user-defined button, as far as users of gui.ss are concerned, is 
not a button% object, but a gui-item structure. So I need some way, in 
the rewritten version of gui.ss, to take a button% (provided by MrEd 
when the button is clicked) and find the corresponding gui-item so that 
I can provide it to the user-supplied callback function. I don't quite 
see how to do this. Can someone help? Many thanks. --PR