MOP is an acronym for Meta-Object Protocol. This is a system with Meta-Classes accessible to users [Kiczales 92, Paepcke 93]. In CLOS terminology, an introspective protocol provides a read only capability (e.g. what is this object's class, give info on this class, etc.) and an intercessory protocol provides a write capability which allows system modification (e.g. add the following method or instance to this class, perform inheritance this way, etc.). Because inheritance can be used to perform differential changes, intercessory protocols allow users to not only define new frameworks but to specialize existing system frameworks differentially without affecting them and their extant objects. Thus, many frameworks can interoperate together simultaneously. This is a good example of object-oriented reuse, since the compiler itself is reused thru specialization to provide new frameworks.
"Reflective" systems are systems with MOPs (not to be confused with reflexive systems, which often refer to systems implemented in terms of themselves, or bootstrapped). Reflective systems are inevitably reflexive (as are most quality compilers), providing a direct program interface to the system.
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