Session 2 HW (ack.py and series.py)

Per [PEP 8](https://www.python.org/dev/peps/pep-0008#maximum-line-length), docstrings should be limited to 72 characters. Consider using a [multi-line docstring](https://www.python.org/dev/peps/pep-0257/#multi-line-docstrings).