What is `last(inf_iter)`. E.g `last(count())`.
The "last" is just a helper function that gets the last value of an iterable. On sequences, it can be written to get the item at index -1 to avoid traversing it. Using it on endless iterables makes no sense.
This makes it clear that is the users job to make sure `it` terminates.
If one call "last" for something that doesn't terminate, an "endless" iterable, well, it's pretty obvious that it won't "end" nicely. It's not the Python job to solve the Entscheidungsproblem. If you call "sorted" on endless iterables, it would behave like "last", doesn't it?
The whole point of this idea is the scan as a generator expression or list/set comprehension that can access the previous iteration output. Reduce/fold is just the last value of a scan, and the scan is still defined when there's no "last value".