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To develop our stop list class we will use a test-driven development (TDD) application of unit testing. In TDD, before we implement a feature (e.g., a method within our stop list class), we first write a test case that, once the new feature is implemented, will ensure it meets specification. The motivating idea for TDD is that the developer should first reflect on a feature's specification and how it will be used in systems of which it is a part.