slot-unbound class instance slot-name => {result}{*}
slot-unbound (class t)
instance slot-name
class---the class of the instance.
instance---the instance in which an attempt was made to read the unbound slot.
slot-name---the name of the unbound slot.
result---an object.
The generic function slot-unbound is called when an unbound slot is read in an instance whose metaclass is standard-class. The default method signals an error
of type unbound-slot. The name slot of the unbound-slot condition is initialized to the name of the offending variable, and the instance slot of the unbound-slot condition is initialized to the offending instance.
The generic function slot-unbound is not intended to be called by programmers. Programmers may write methods for it. The function slot-unbound is called only indirectly by slot-value.
If slot-unbound returns, only the primary value will be used by the caller, and all other values will be ignored.
The default method on slot-unbound signals an error of type unbound-slot.
section slot-makunbound [Function]
An unbound slot may occur if no :initform form was specified for the slot and the slot value has not been set, or if slot-makunbound has been called on the slot.
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