- head at this stage and the result accumulated so far, from folding the same
- function (and a base value) to the tail at this stage...and then pass the result
- of doing so leftward along the rest of the list.
+ head at this stage and the result accumulated so far (from folding the same
+ function, and a base value, to the tail at this stage)...and then pass the result
+ of that application to the embedding, more leftward computation.