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opaque type proposal

OK, I think I buy your arguments for single inheritance.  So how about
this: MAKE-RECORD-TYPE takes as an additional argument a descriptor for
the parent type, or #f if there is to be none.

About the identification argument to MAKE-RECORD-TYPE, the reason I
suggested it be a symbol was just to avoid overspecification: objects of
other types could mean different things.  E.g. at some point that
argument could (upward-compatibly) become some kind of handler procedure
for the record type.

The identification argument should probably be optional, or perhaps not
exist at all.  I don't know.

By the way, the object returned by MAKE-RECORD-TYPE ought to be called a
"record type descriptor", or anything other than "record type".  Calling
any object a "type" would be a bad precedent and would frustrate the
efforts of those using Scheme for teaching and designing statically
typed languages.

I would oppose anything resembling TYPE-OF, since whenever you have
subtyping or polymorphism, there's no such thing as THE type of an
object.  I don't see why it would be useful either, since you can always
put type-specific information (like generic operation handlers)
somewhere else; e.g. in the case of records, you can put it in the type