+ "Define internal accessors for instances of the luna class CLASS-NAME.
+
+Internal accessors are macros to refer and set a slot value of the
+instances. For instance, if the class has SLOT, macros
+CLASS-NAME-SLOT-internal and CLASS-NAME-set-SLOT-internal are defined.
+
+CLASS-NAME-SLOT-internal accepts one argument INSTANCE, and returns
+the value of SLOT.
+
+CLASS-NAME-set-SLOT-internal accepts two arguemnt INSTANCE and VALUE,
+and sets SLOT to VALUE."