Metaobject Protocol
Outside of the ANSI Common Lisp standard, there is a widely implemented extension to CLOS called the Metaobject Protocol (MOP). The MOP defines a standard interface to the underpinnings of the CLOS implementation, treating classes, slot-descriptions, generic-functions and methods themselves as instances of metaclasses, and allows the definition of new metaclasses and the modification of all CLOS behavior. The flexibility of the CLOS MOP prefigures aspect-oriented programming, which was later developed by some of the same engineers, such as Gregor Kiczales. The MOP defines the behavior of the whole object system by a set of protocols. These are defined in terms of CLOS. Thus it is possible to create new object-systems by extending or changing the provided CLOS functionality. The Book The Art of the Metaobject Protocol describes the use and implementation of the CLOS MOP.
The various Common Lisp implementations have slightly different support for the Meta-Object Protocol. The Closer project aims to provide the missing features.
Read more about this topic: Common Lisp Object System