+/* --- These are determined from the window size --- *
+ *
+ * Given a %$k$%-bit exponent, I expect to do %$k/2$% multiplies if I use the
+ * simple way. If I use an n-bit sliding window, then I do %$2^n$%
+ * multiplies up front, but I only do %$(2^n - 1)/2^n k/n$% multiplies for
+ * the exponentiation. This is a win when
+ *
+ * %$k \ge \frac{n 2^{n+1}}{n - 2}$%
+ */