Let's start with this function. I have changed my mind - I think this is complicated enough that the test should be used as a _filter_ rather than an _assertion_. How many pairs are generated by this function? How many should there be? If this is the full graph of X and Y:
![xy100](https://f.cloud.github.com/assets/177694/1799180/9b059dd2-6b8a-11e3-9e35-6ebc8d3d93d6.jpg)
... then can you draw me a graph of the acceptable points?