Sat Aug 8 12:33:11 CEST 2009

closures and values

So.. It's probably best to move closure_pack to the start of step(),
and unpack before adding to the environment.  It's a bit awkward the
way it is now.

Let's make it such that:

      - on step() entry, the closure is unpacked to perform reduction.

      - if there is no reduction, the value is packed.