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In Project Builder, you will see that Foo.h and Foo.m have been added to the project. Most programmers would put these files under the Classes group (Figure 2.24). Drag the files into the Classes group if they aren't there already. Make sure that the circle to the left of the filename doesn't disappear. The file will be compiled into the application only if the circle is there. By clicking on the circle, you can make it appear and disappear.