[racket] Notification when window is closed

On Sun, Aug 5, 2012 at 2:27 PM, Robby Findler
<robby at eecs.northwestern.edu> wrote:
> My first guess is that the right way to do this is to use custodians
> to do this shutdown, not closing the window (and perhaps make closing
> the window shutdown the custodian, depending). Generally speaking,
> custodians are the abstraction that Racket uses to manage the clean
> shutdown of external state (like network connections, open files, open
> windows, etc).

Sounds like a good idea.  Is it possible to register a finalizer in a
custodian?  Or something similar that allows me to call an FFI function.