Common Lisp the Language, 2nd Edition
The second step initializes the newly added local slots and performs any other user-defined actions. This step is implemented by the generic function update-instance-for-redefined-class, which is called after completion of the first step of modifying the structure of the instance.
The generic function update-instance-for-redefined-class takes four required arguments: the instance being updated after it has undergone the first step, a list of the names of local slots that were added, a list of the names of local slots that were discarded, and a property list containing the slot names and values of slots that were discarded and had values. Included among the discarded slots are slots that were local in the old class and that are shared in the new class.
The generic function update-instance-for-redefined-class also takes any number of initialization arguments. When it is called by the system to update an instance whose class has been redefined, no initialization arguments are provided.
There is a system-supplied primary method for the generic function
update-instance-for-redefined-class whose parameter specializer for
its instance argument is the class standard-object. First this
method checks the validity of initialization arguments and signals an
error if an initialization argument is supplied that is not declared
Then it calls the generic function
shared-initialize with the following arguments: the instance,
the list of names of the newly added slots, and the initialization
arguments it received.