Quiz: Primes in the BCL...

I played around a little with computing prime numbers last week, fun stuff but more on that for a another day…. When I discussed some thoughts on the subject with devs on the BCL team I found out (to my surprise) that we actually have code in the BCL for computing prime numbers… Now it is not directly callable publicly, but the source code is in rotor…

Anyone know where? Extra credit if you can explain why we use primes (and need to compute them)… even more extra credit if you can find a small bug in that code…