Session02

It's a good thought to want to reuse functions. However, in this case, I'd argue that this is undesirable. The intent is that `sum_series()` be a more general function than `fibonacci()` and `lucas()`. Your implementation of `sum_series()` would produce the same output without lines 26 - 29. Including those lines makes your function definition longer than it needs to be and also makes it dependent upon working implementations of those functions.