+(defun update-user-data (id object)
+ (check-type id fixnum)
+ (multiple-value-bind (user-data exists-p) (gethash id *user-data*)
+ (cond
+ ((not exists-p) (error "User data id ~A does not exist" id))
+ (t
+ (when (cdr user-data)
+ (funcall (cdr user-data) (car user-data)))
+ (setf (car user-data) object)))))
+