(cond ((>= severity 4) :remove)
((>= severity 3) t)
(t nil)))
This keyword controls whether class accessor functions are kept in their slot-definition objects. Removing them allows unreferenced functions to be deleted.
If it is nil it ensures all accessors are kept.
If it is non- nil , class accessors which are never referenced are deleted.
If it is :remove , all class accessor functions are removed from their slot descriptions.
In general, accessors may be safely removed. However, if your application needs to examine the slots of class instances, you need to retain them.