[plt-scheme] Clas override, MrEd question...

I'm looking for/implementing a filtered choice/list box, which restricts 
the available choices to those that match a filter string.  I haven't 
seen any available widgets that do this, so I've begun hacking together 
a choice% subclass.  I've captured keyboard events, to create the filter 
string, but I cannot override the append method, and call the superclass 
append method from within it.  I would like this to happen within the 
one widget (not really after a seperate text field, although I may have 
to go that way).  Any suggestions would be greatly appreciated.

Thanks

Rian Douglas