check-in working versions of series.py and ack.py; need more work on the...

... asserts
While this `for` loop does work, it's rather confusing to read because you're changint the value bound to the symbol `n`. The `n` in `range(2, n + 1)` is bound to the value of the argument that `fibonacci()` is called with while the `n` in `for n in` binds `n` to each successive value in the list created by `range(2, n + 1)`. This code would be much clearer if you chose a different symbol for the iteration. Something like:

    for i in range(2, n + 1):