Sat Dec 24 10:18:37 EST 2011

Why binary trees as primitive structure?

Because it reflects the state composition of state space model
Arrow composition, both parallel and serial.

That makes perfect sense.  However, I still see no real reason to
flatten such datastructures in Pack and App.  Though in Lambda and
Unpack it seems to make sense: they are *really* unstructured:
replaced by individual variable names.  Let's take that as the reason:
a list is just a convenient way to represent a set, and it has just
the right amount of structure to also fix memory allocation in the
lower layers.