Common Lisp the Language, 2nd Edition

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### 28.1.10.4. Extensions

There are two allowed extensions to class redefinition:

- The Object System may be extended to permit the new class to be an instance of a metaclass other than the metaclass of the old class.

- The Object System may be extended to support an updating process when either the old or the new class is an instance of a class other than `standard-class` that is not a built-in class.