Nice to see the arrival of the prod() function.

Just as for the built-in pow(x, y[, z]) function it would be very useful to have an optional argument z for computing products modulo z. Typical use case in cryptography would be:

prod((pow(x, y, z) for x, y in zip(g, s)), z)

to compute the product of all (potentially many) g[i]**s[i]'s modulo z. 

And, just as with the use of pow(), the intermediate values for prod() may in general grow quickly, hence modular reduction is essential to limit time and space usage.