Common Lisp the Language, 2nd Edition
In order to make an instance conform to the class , local slots specified by the class that are not specified by the class are added, and local slots not specified by the class that are specified by the class are discarded.
The values of local slots specified by both the class and the class are retained. If such a local slot was unbound, it remains unbound.
The values of slots specified as shared in the class and as local in the class are retained.
This first step of the update does not affect the values of any shared