Bugs in ispower_moduli?

[lgoettgens]

I was curious what these vectors are used for. Turns put the code is buggy: the code in line 391 below checks for divisibility by 3 instead of 7... oops.

Originally posted by @fingolfin in #1749 (comment)

[fingolfin]

I suspect the code is still correct but perhaps less efficient.

The lack of any comments explaining what's going on (which admittedly is not that hard to guess) or more importantly, why those specific numbers have been chosen, doesn't help :-(