Creating new kinds of collections

We'll look at some extensions we might make to Python's built-in container classes. We won't show an example of extending each container.

We'll pick an example of extending a specific container and see how the process works:

1. Define the requirements. This may include research on Wikipedia, generally starting here: http://en.wikipedia.org/wiki/Data_structure. Designs of data structures often involve complex edge cases around missing items and duplicate items.
2. If necessary, look at the collections.abc module to see what methods must be implemented to create the new functionality.
3. Create some test cases. This also requires careful study of the algorithms to ensure that the edge cases are properly covered.
4. Write code based on the previous research steps.

We need to emphasize the importance of researching the fundamentals before...