+;;; AMOP says that CLASS-SLOTS signals an error for unfinalized classes.
+(defmethod class-slots :before ((class slot-class))
+ (unless (class-finalized-p class)
+ (error 'simple-reference-error
+ :format-control "~S called on ~S, which is not yet finalized."
+ :format-arguments (list 'class-slots class)
+ :references (list '(:amop :generic-function class-slots)))))