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I did more code review, testing, and timing.  The only
change in this new patch (long_mont2.diff) is a couple
"int"s were changed to "digits"s, and it's against CVS head.

As far as testing, I used the random module and GMPY to
check it on ~3 million random input values.  That's about an
hour of testing.  I'll leave the tests running for a few
days and see if anything crops up.

As far as timing, I updated the benchmarks with a new
machine (OpenBSD):
http://trevp.net/long_pow/
On 3 different machines, Montgomery gives a speedup of 2x,
3x, and 4x.  That dwarfs what we've done so far, so I'm
crossing my fingers for 2.4.  Let me know if I can explain
or improve the code, or anything..  

(The below crypto library comes with a "book" which has an 
explanation of Montgomery I found helpful):
http://math.libtomcrypt.org/download.html