redefining Common Lisp functions

[masinter.PA@Xerox.COM]

Everyone (including Guy Steele!) seems to be challenging me to quote
"chapter and verse" in Guy Steele's book supporting my assertion that it
is an error to redefine any of the predefined Common Lisp functions and
macros.  (The appropriate verse which claims that it is an error to
redefine one of the predefined special forms with DEFUN having been
found.)

Rather than attempting to divine meaning in CLtL, lets focus on some
related but different questions:

1. (Current practice) In your favorite Common Lisp implementation, which
of the predefined Common Lisp functions, macros, and special forms can
be safely redefined without causing unpredictable behavior somewhere
else in the system? 

2. (Future standards) Which of the predefined Common Lisp functions,
macros, and special forms do you think *should* be redefinable.

3. (More constrained redefining) Presuming the CLOS, which of the
predefined Common Lisp functions should be specializable for
user-defined classes? How should such specialization affect the rest of
the behavior of Common Lisp?