+ * Finally, we consider general permutations. These can be implemented using
+ * Beneš networks. Pick some index bit number %$i$%. By applying a swizzle
+ * with a shift by %$2^i$% to the inputs, and another to the outputs, we can
+ * reduce the problem to finding two independent permutations, one affecting
+ * bits whose index has bit %$i$% clear, and the other affecting bits whose
+ * index has bit %$i$% set. This doesn't sound so helpful, except that (a)
+ * the smaller permutations can each be implemented in the same way, and (b)
+ * they can be performed in parallel. Small Beneš networks can be
+ * constructed by hand, but computer assistance is useful for larger ones;
+ * there are some utilities in `utils/benes.lisp'.
+ *