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Lesson 2, “Extending Existing Class Capabilities,” covers the subject of Categories, a technology that enables developers to extend the capabilities of existing classes without altering the source code of those classes and without using inheritance. Viewers learn how to declare categories and how to implement those category methods. Then, the lesson covers a design pattern with categories. Viewers also learn how to implement protocols with categories and how to use anonymous categories. Finally, the lesson covers the gotchas to watch out for and avoid when working with categories.