Session02

Files series.py and ack.py are complete with tests
If you have to define a symbol, but don't actually intend to use it for anything, like is the case with ``i``, it's convention to use an underscore:

    for _ in range(n):
        # do stuff that doesn't reference _

This is a minor detail and certainly isn't required, but I wanted to make sure you're aware in case you see it in anyone else's code.