[racket] Review request: Partially expanding to make "blue boxes"

> [^1]: The local-expand docs actually say: "This procedure must be
> called during the dynamic extent of a syntax transformer application
> by the expander or while a module is visited". The latter possibility
> sounds promising, but I don't understand how I would insinuate myself
> into the process of visiting a module. Would this be via overriding
> current-compile then chaining to the original?

To answer my own question:

>From experimentation, no.

The time when `current-compile` is called doesn't qualify as "while a
module is visited" -- `local-expand` gives the usual error:
"local-expand: not currently transforming".