Colin wrote: "What would b really cool though, (and I don't know if Martin's
work allows for this) would be user-definable immediates. Adding a tag bit
allows for 2 more immediate classes. With the right primitives, one could
define a class, designate it as immediate and write methods that manipulate
the bits of the object references, much the way SmallInteger does. There
would probably have to be some machinery similar to the way compact classes
are managed."

I had this same idea about 10 years ago.  However, it would seem to be even
more useful in a 64-bit architecture (where each OOP is a 64-bit value.)
Folding the implementation of both a 64-bit OOP architecture and
programmer-definable immediate types into the same project would probably be
a big win.

--Alan