replace treemap with a balanced tree
add a lazy forward iterator to TreeMap
make Eq implementation O(n)
clean up equality code a bit
rm extra newline
remove 'TODO' from the list of future improvements
fix API of union
implement set difference
range search would be nice
add scaffolding for symmetric_difference/union
make TreeSet tests a bit more paranoid
implement set union
implement symmetric_difference
fix bug in union implementation (missing return)
docstring/comment fixes
add TreeSetIterator
cleanup
make is_disjoint O(n+m) instead of O(n*log(m))
make intersection O(n+m) instead of O(n*log(m))
make is_superset/is_subset O(n+m) instead of O(n*log(m))