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 *************************************************************** #define private public dan@milliways.org Worcester Polytechnic Institute and The Restaurant at the End of the Universe ***************************************************************