We have to make sure that the small number of things that we change are changed correctly. On the negative side, well, that isn’t the only thing we have to concentrate on. We have to figure out how to preserve the rest of the behavior. Unfortunately, preserving it involves more than just leaving the code alone. We have to know that the behavior isn’t changing, and that can be tough. The amount of behavior that we have to preserve is usually very large, but that isn’t the big deal. The big deal is that we often don’t know how much of that behavior is at risk when we make our changes. If we
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