write initial correctness tests

[orbeckst]

Once you have added your initial code (#7) you should write tests that produce results that you know to be correct.

Once you start refactoring your code, the tests will remain the same (at least the reference values ("fixtures") that they check against). This will provide you the confidence that your code changes do not introduce bugs. (In truth, you'll likely discover bugs in your old code and new bugs will creep in but still, your initial tests will be the foundation on which you're doing your work).