scans.txt

Blelloch
    Upsweep condition is that at the end of step i it's the case that each block of size 2^i has the sum of all of the elements in the block in the last position of the block
    Downsweep condition is that if we copy the sum of all the elements in the previous block to the last position in a block we'll correctly compute the sum

Hillis-Steele
    Condition is that after step i each element contains the sum of the elements 2^i - 1 away from it, so blocks of size 2^i are correct.