-BN_is_prime() tests if the number B<a> is prime. This is done by
-performing a Miller-Rabin probabilistic primality test with B<checks>
-iterations. If B<checks == BN_prime_check>, it uses a number
-of iterations that yields a false positive rate of at most 2^-80 for
-random input.
+BN_is_prime() and BN_is_prime_fasttest() test if the number B<a> is
+prime. The following tests are performed until one of them shows that
+B<a> is composite; if B<a> passes all these tests, it is considered
+prime.
+
+BN_is_prime_fasttest(), when called with B<do_trial_division == 1>,
+first attempts trial division by a number of small primes;
+if no divisors are found by this test and B<callback> is not B<NULL>,
+B<callback(1, -1, cb_arg)> is called.
+If B<do_trial_division == 0>, this test is skipped.
+
+Both BN_is_prime() and BN_is_prime_fasttest() perform a Miller-Rabin
+probabilistic primality test with B<checks> iterations. If
+B<checks == BN_prime_check>, a number of iterations is used that
+yields a false positive rate of at most 2^-80 for random input.