If this function returns 0,
a is definitely not prime. If it returns 1, then a is "probably" prime. If it
returns 2, then a is surely
prime. Reasonable values of
reps vary from 5 to 10 (default being 10); a higher
value lowers the probability for a non-prime to pass as a
"probable" prime.
The function uses Miller-Rabin's probabilistic test.