[plt-scheme] Similarity checking

Hi all,

Maybe someone has developed a code or concept that I could use.
I teach quite a few students. 
They have to write tests and hand in homeworks. It seems to me
that nowadays it is a tendency, that they track down who has the
same problem and they share the results, the programs. 
(Recently I have seen quite a few high tech to do this.) 
They are smart to change for example the variable names and
some bits, but of course not the whole program. So I would like
some help from a program and check automatically the handed in
programs for plagiarism or similarity.
Is there any code which can compare Scheme programs? Or maybe 
compares programs with a subset of Scheme? 
Has anybody heard or thought about it?

I am thinking along the line to parse in the program to have some
binary tree, but I am not sure how to do "fuzzy"-style comparison
on them.

I would appreciate any suggestion.

Peter Ivanyi