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Unbound Slots

NOTE:  The name
has been changed to

Not all slots in an instance need to be bound.  This can happen if a
slot has no :initform, an has not been set, or if the user has called
slot-makunbound.  Referencing an unbound slot of an instance of a
standard class causes the generic-function slot-unbound to be invoked.
The arguments to slot-unbound are the same as the arguments to
slot-value-using-class, that is, the class of the instance whose slot
was accessed, that instance, and the name of the slot.  

The generic function slot-unbound is not intended to be called
by programmers.  Programmers are expected to write methods for it.
The function slot-unbound is called only by the function

   slot-unbound class instance slot-name   (generic function)
The slot-unbound method on standard-class signals an error.