[plt-scheme] tree control and drag & drop support

Is there a tree widget for MrEd?  I found one in MrEd Designer, as you'll 
see below [1].  However, it doesn't seem too general.  Anything else out 
there?  Is there something in the underlying widget library that I could 
expose?

Also, I've seen that there is support for accepting a dropped file from the 
desktop system.  Is there a more general facility for defining drag & drop 
sources and sinks in your program?

Thanks,
Tom

[1]
http://mreddesigner.lozi.org/index.php