Boolean tasks

I added ackerman.py and series.py.
This definition for `sum_series()` is causing a `RuntimeError` because it's reaching the "maximum recursion depth." Can you explain what you are trying to accomplish with this function? We might be able to re-write this function so that it does what you intend without hitting the recursion limit. Alternatively, you could replace it with your previous version as I believe it worked correctly.