[racket] instantiating multiple sandboxes with gui's

> thread cells are subject to GC and that's fine. The real issue is that I
> might write a program that does (effectively) this:
>
>  > (let l()(l))
>
> and then I'm stuck.

Good point, but that's not a problem. A break returns to read-eval-print-loop.

Example transcript:

> (let l () (l))
C-cC-c
; user break
> (displayln "broken but alive")
broken but alive
>

(The double Control-C just being necessary as usual in an Emacs comint-buffer.)