[racket] match and debug in Advanced Student

I never use the Debug button, but I let my students start using Advanced 
Student today, and of course they tried it out.

The following program induces a strange error when the Debug button is 
pressed.

(match 3
   [`(,x) 3]
   [x 3])

The error is:
internal error: skipped binding is used

after which the program hangs, though it can be stopped with the Stop 
button. --PR