start cleanup bits of the reduce_sum code. Mostly to get an idea of what I want it to look like
start cleanup bits of the reduce_sum code. Mostly to get an idea of what I want it to look like

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math