[plt-scheme] Code review: garden fence encryption

Hi,

I don't know whether this is the correct list, but of all Scheme
communities this one seemed to be the best fit for this mail.

Anyway, I had the idea of implementing a simple encryption algorithm
that even children can use, as a kind of exercise in writing functional
code in Scheme.

Attached is a version of that script as it is quite long. I tried to
write enough documentation so that both the garden fence algorithm as
well as my implementation are understandable.

It would be good if some more experienced Schemers could take a look at
the code and give me some tips on how I could improve it. I usually
learn quite a lot from such reviews and value the input of other people.

In case the attachment is a problem, the current version can also be
found in my sandbox repository:
<http://xivilization.net/hg/sandbox/file/tip/gardenfence.scm>

regards,
Marek
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