Modifying the Structure of the Instance (ANSI and GNU Common Lisp Document)

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#### 7.2.1 Modifying the Structure of the Instance

In order to make the instance conform to the class C_{to}, local slots specified by the class C_{to} that are not specified by the class C_{from} are added, and local slots not specified by the class C_{to} that are specified by the class C_{from} are discarded.

The values of local slots specified by both the class C_{to} and the class C_{from} are retained. If such a local slot was unbound, it remains unbound.

The values of slots specified as shared in the class C_{from} and as local in the class C_{to} are retained.

This first step of the update does not affect the values of any shared slots.