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ANSI Common Lisp  4 Types and Classes 4.3 Classes 4.3.6 Redefining Classes
4.3.6.1 Modifying the Structure of Instances
The first step modifies the structure of instances of the redefined
class to conform to its new class definition.
Local slots specified
by the new class definition that are not specified as either local or
shared by the old class are added, and slots
not specified as either
local or shared by the new class definition that are specified as
local by the old class are discarded.
The names of these added and discarded
slots are passed as arguments
to update-instance-for-redefined-class
as described in the next section.
The values of local slots specified by both the new and old
classes are retained. If such a local slot was unbound,
it remains unbound.
The value of a slot that is specified as shared in the old
class and as local in the new class is retained. If such
a shared slot was unbound, the local slot is unbound.
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