venn diagram prototype

Hi,
as my first attempt to contribute to Khan Academy I've tried to develop an exercise on Venn diagrams. It's far from ideal, but I'd rather get some feedback before it gets even more messy.

I've read some wiki pages on making an exercise, but it's a lot of information to process, so feel free to point me to any documentation, if it looks like I've missed it.

TODO:
- place intersection numbers in the middle of intersection 
- generate subset formulas without recursion (sometimes overflows)

Future plans:
- venn diagram 0.5: make intersections selectable (by clicking); task is just to select the right ones.
- add hints: use selectable intersections - highlight them
And many more...

Thanks for any feedback
Vít