Immed. values, classes w/o new(), non-marshallables

Posted to Rubyforum, I thought I'd try here too. I'm trying to
understand what, if anything, these three groups of slippery objects
have in common:

- Immediate values (Fixnum, SYmbol, true, false, nil), that
ObjectSpace doesn't see

- objects e.g. literal strings that don't construct thru Class#new
(meaning they don't run new() in overriden form or super() form

- objects that can't be marshalled: proc/method objects, singleton and
IO objs, etc, see page 535 of pickaxe 2.