The Common Lisp Object System, an object-oriented programming language embedded in and somewhat integrated into Common Lisp.

It is a class-based system, and the first widely-used language to have multiple-dispatch, instance-specific specializers, and method composition. A few artifacts of the fact that it was a late addition to Common Lisp are that it organizes methods into generic functions, a term for Lisp functions which can specialize on their arguments. So it's designed to look like the pseudo-functional style of Lisp while embracing object-orientation.

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