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A software system is the realization of a set of concerns. One of the central premises of object-oriented programming is that each concern should be implemented as a separate module. However, there are certain system-wide concerns, such as logging, security, performance, and so forth, that often need to be addressed in many, if not all of the modules. Consequently, the code to handle these system-wide concerns may be mixed in with the core logic of a huge number of modules, resulting in lower productivity, poor quality, and systems that are hard to evolve.