Common Lisp the Language, 2nd Edition
Methods for the generic function reinitialize-instance may be defined to specify actions to be taken when an instance is updated. If only :after methods for reinitialize-instance are defined, they will be run after the system-supplied primary method for initialization and therefore will not interfere with the default behavior of reinitialize-instance.
Methods for shared-initialize may be defined to customize class
redefinition (see section 188.8.131.52).