Class Precedence List:
unbound-slot, cell-error, error, serious-condition, condition, t
The object having the unbound slot is initialized by the :instanceinitialization argument to make-condition, and is accessed by the function unbound-slot-instance.
The name of the cell (see cell-error) is the name of the slot.
cell-error-name, unbound-slot-object, Section 9.1 (Condition System Concepts)