Tue Apr 2 15:52:17 EDT 2013

Control rate ops

Maybe this should just be a different type signature?  The setup has
"extra state" that can be used by the update as input.

setup:   (s0, s1, i) -> (s0, s1)
update:  (s0, s1, i) -> (s0, o)

Probably not necessary yet.  The current scheme can do the above if
necessary by setting s = s0 x s1.