Jonathan returns from his meeting, and Mike quickly walks him through the Sequencer. Then they start working on the Writer.
Once again, notice how careful they are about testing. Notice how the design of the Writer improves before any of the Writer code is written, simply because of the effort they put into the tests.
These three tomatoes are pretty straightforward, but I want you to pay special attention to they way they refactor and clean up the tests: they remove redundancy wherever they see it.
Notice also the discipline they use with their mocks. They aren't afraid to write their own. They seem to have no need for a mocking framework.