initial commit
still todo:
- tests
- LVarSet
- https://github.com/aphyr/meangirls#g-set
- https://github.com/aphyr/meangirls#2p-set
- https://github.com/aphyr/meangirls#lww-element-set
- https://github.com/aphyr/meangirls#or-set
- https://github.com/aphyr/meangirls#max-change-sets