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Carl, I really don't like the idea of having forms which return
"undefined" values unless that can be enforced by the compiler
and the interpreter. If you can convince everyone to have
SETQ, SETF, ney, all side-effecting primitives to return
SI:+INTERNAL-**UNDEFINED-FOOBAR** then I think you would have
better grounds for having SETF's return value be undefined.

I know that mixing side effect producing expressions with expressions
which are evaluated for value can lead to some *REALLY* grotesque
and difficult to read programs, <A classic example of which is
the macsyma source code for solving quibic equations>, 
*but*, it doesn't have to.

As HIC pointed out, even the maclisp compiler optimizes away the
lambda needed to save the value to return just in case it is