IndexableSkiplist - Segmentation fault - 💥
**python**: 3.5.2
**pandas**: 0.19.1
**system**: Mac v 10.12.1 

```python
>>> from pandas._window import IndexableSkiplist
>>> sl = IndexableSkiplist()
>>> len(sl)
0
>>> list(sl)
```
**Segmentation fault: 11** :collision: