[plt-scheme] How to use a custom procedure as read?

Hi,
I'm sorry to ask this question, since it has been asked before, but I could
not make any sense of it at all.
My query is fairly simple, and after an hour or two of searching and trial
and error, I hope I can get the answer an easier way here!

Looked in the reference, this seems to be what I want, but all I'm not sure
exactly how to use it.
http://docs.plt-scheme.org/syntax/reader-helpers.html#(part._module-reader)<http://docs.plt-scheme.org/syntax/reader-helpers.html#%28part._module-reader%29>

Read this and seemed to make sense, but not sure how to apply it to my
problem.
http://www.htus.org/Book/Staging/how-to-use-modules/

I believe the answer is in here, but again, I haven't been able to get it
working.
http://groups.google.com/group/plt-scheme/browse_thread/thread/8aabfc15e0a392df/8a67d42dfcdadcae?lnk=gst&q=reader#8a67d42dfcdadcae

Problem:
I've got a Scheme procedure which I would like to temporarily use as the
read procedure, eg.

(swap-the-reader-in)
... code using custom reader ...
(swap-the-reader-out)
... normal reader again ...

My MzScheme version is 372. I'd be really grateful for an example of how to
temporarily use a custom procedure as the reader function.
Thanks, Al
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