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

... asserts
Two things: 

 1. As mentioned above, at this point the symbol `n` is bound by the `for` loop to the current element of the sequence the loop is iterating over. Adding one to `n` at the bottom of the `for` loop code block is unnecessary (and not particularly helpful) because the loop immediately rebinds `n` to the next value in the sequence. 

 1. This line would be more Pythonic if you used in place assignment. It's not vital, and, in this case, it only saves you a single character, but it's a good habit to get into.