[plt-scheme] [scribble] boxing/highlighting in scheme code

Hi,

Is there a way when typesetting a block of scheme code to insert a box or a shading/highlighting on a portion of it, in order to attract the attention of the reader on that portion?

For instance, something like:

@schemeblock[
(define (foo x)
  (if (null? x) '()
      @highlight[
      (if (eq? (car x) ...)
          ...
          ...)
      ]))

would highlight the "what-we-do-when-the-arg-is-a-non-empty-list" part of the code.

Thanks,

-- Éric