&optionalnew-value → result
The generic function slot-missing is invoked when an attempt is made to access a slot in an object whose metaclass is standard-class and the slot of the name slot-name is not a name of a slot in that class. The default method signals an error.
The generic function slot-missing is not intended to be called by programmers. Programmers may write methods for it.
The generic function slot-missing may be called during evaluation of slot-value, (setf slot-value), slot-boundp, and slot-makunbound. For each of these operations the corresponding symbol for the operation argument is slot-value, setf, slot-boundp, and slot-makunbound respectively.
The optional new-value argument to slot-missing is used when the operation is attempting to set the value of the slot.
If slot-missing returns, its values will be treated as follows:
The set of arguments (including the class of the instance) facilitates defining methods on the metaclass for slot-missing.