+ /* --- Figure out how large the smaller primes should be --- *
+ *
+ * We want them to be `as large as possible', subject to the constraint
+ * that we produce a number of the requested size at the end. This is
+ * tricky, because the final prime search is going to involve quite large
+ * jumps from its starting point; the size of the jumps are basically
+ * determined by our choice here, and if they're too big then we won't find
+ * a prime in time.
+ *
+ * Let's suppose we're trying to make an %$N$%-bit prime. The expected
+ * number of steps tends to increase linearly with size, i.e., we need to
+ * take about %2^k N$% steps for some %$k$%. If we're jumping by a
+ * %$J$%-bit quantity each time, from an %$N$%-bit starting point, then we
+ * will only be able to find a match if %$2^k N 2^{J-1} \le 2^{N-1}$%,
+ * i.e., if %$J \le N - (k + \log_2 N)$%.