class—a class designator.
make-instances-obsolete has the effect of
initiating the process of updating the instances of the
class. During updating, the generic function
update-instance-for-redefined-class will be invoked.
The generic function
make-instances-obsolete is invoked
automatically by the system when
defclass has been used to
redefine an existing standard class and the set of local
slots accessible in an
instance is changed or the order of slots in storage is changed. It
can also be explicitly invoked by the user.
If the second of the above methods is selected, that
update-instance-for-redefined-class, Section 4.3.6 (Redefining Classes)