Their class works beautifully. They decide it's time to integrate their new code. They run the tests for the entire system. They have a good laugh while they load the missing pieces. Once done, they rerun the tests and pass with flying colors.
Developers integrate new code into the system several times per day.
Duff agrees and they start a new pairing session. He has been working by himself for about an hour, so he walks Jeff through what he's done. Jeff sees a prime target for refactoring to make a class much simpler. They attack that problem first. By the end of the day, they have finished the refactoring, created three new classes, and tested them.