MAINT: improve readablility of cross and improve test coverage

What are the ` - 0` and the `0 -` supposed to mean? I think the comment would be much more informative without them, it may even be worth spelling the whole operations out like:

```
# cp0 = a1*b2
# cp1 = -a0*b2
# cp2 = a0*b1 - a1*b0
```