1. Specify and verify the upsweep method. You can begin with a slightly
   simpler requirement that the last array cell contains the sum of the
   whole array in the post-state.
2. Verify both upsweep AND downsweep - prove that the array cells
   contain appropriate prefix sums in the post-state.

If a general specification is not possible with your tool, assume the
length of array is 8. 
