-;;; T: means everything so far is the class T
-;;; STANDARD-INSTANCE: seen only standard classes
-;;; BUILT-IN-INSTANCE: seen only built in classes
-;;; STRUCTURE-INSTANCE: seen only structure classes
-;;; CONDITION-INSTANCE: seen only condition classes
+;;; T: means everything so far is the class T.
+;;;
+;;; The above three are the really important ones, as they affect how
+;;; discriminating functions are computed. There are some other
+;;; possible metatypes:
+;;;
+;;; * STANDARD-INSTANCE: seen only standard classes
+;;; * BUILT-IN-INSTANCE: seen only built in classes
+;;; * STRUCTURE-INSTANCE: seen only structure classes
+;;; * CONDITION-INSTANCE: seen only condition classes
+;;;
+;;; but these are largely unexploited as of 2007-05-10. The
+;;; distinction between STANDARD-INSTANCE and the others is used in
+;;; emitting wrapper/slot-getting code in accessor discriminating
+;;; functions (see EMIT-FETCH-WRAPPER and EMIT-READER/WRITER); it is
+;;; possible that there was an intention to use these metatypes to
+;;; specialize cache implementation or discrimination nets, but this
+;;; has not occurred as yet.