Rosing's Elliptic curve documentation question

I was reading Mike Rosing's documentation for his elliptic curve
system from the Cypherpunks FTP site.  In it, he uses the notation:
sum a_i*b^2^i
	Now, like a good little Calculus 2 student, I assumed this must be a
regular summation.  But if so, which is the index of summation, and
what are the bounds of summation?  Someone want to give me a clue as
to what this notation means?
						Dan Bailey
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#define private public						dan@milliways.org
Worcester Polytechnic Institute and The Restaurant at the End of the Universe
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