-While candidates for p are being tested, B<callback(1, j++, cb_arg)>
-is called in the inner loop of the Miller-Rabin primality test, then
-B<callback(0, counter, cb_arg)> is called when the next candidate
-is chosen.
+Before a candidate for p (other than the first) is generated and tested,
+B<callback(0, counter, cb_arg)> is called.
+
+=item *
+
+When a candidate for p has passed the test by trial division,
+B<callback(1, -1, cb_arg)> is called.
+While it is tested by the Miller-Rabin primality test,
+B<callback(1, i, cb_arg)> is called in the outer loop
+(once for each witness that confirms that the candidate may be prime).
+i is the loop counter (starting at 0).