Common Lisp the Language, 2nd Edition
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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.
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