The memoization occurs in the fact that we are "remembering" the previous value (pre_sum). It's a little different in that we aren't memoizing/caching all previ

The memoization occurs in the fact that we are "remembering" the previous value (pre_sum). It's a little different in that we aren't memoizing/caching all previous values, but we are still caching the last computed values aka the "tail".