Actually, no, for sum you need three things: how to start (0), how to accumulate (plus) and how to combine two reductions (plus again). This is exactly what fo

Actually, no, for sum you need three things: how to start (0), how to accumulate (plus) and how to combine two reductions (plus again). This is exactly what fold doe Equally, for filter, it's empty list, conj or not and concat. (Just to demonstrate that sometimes the combine is different from the accumulate stage.)