Implementation of sets using sorted lists

This script implements set operations using sorted lists as the underlying data structure.

Advantages:
- Space savings -- lists are much more compact than a dictionary based implementation.
- Flexibility -- elements do not need to be hashable, only __cmp__ is required.
- Fast operations depending on the underlying data patterns.
- Non-overlapping sets get united, intersected, or differenced with only log(N) element comparisons.Results are built using fast-slicing.
- Algorithms are designed to minimize the number of compares which can be expensive.
- Natural support for sets of sets. No special accomodation needs to be made to use a set or dict as a set member, but users need to be careful to not mutate a member of a set since that may breaks its sort invariant.
Disadvantages: 
-  Set construction uses list.sort() with potentially N log(N) comparisons.
- Membership testing and element addition use log(N) comparisons. Element addition uses list.insert() with takes O(N) time.