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Another EQ problem

	First, let me give Barry Smith (bds@mitre-bedford.ARPA) BIG THANKS
for posting his "2 Problems" mail with the eq-test program.  It solved a
bug by analogy, and saved me *much* time and aggravation.
	I think that the following is a bug that should be changed:
	I have some code that does surgery (setf's of nth's) on lists in
a Defstruct.  Since some of the fields are identical, the lists are eq.
This means (obviously, once you know about it) that if you change slot A,
slot B is smashed too!
(defstruct foo
	(a '(0.0 0.0))
	(b '(0.0 0.0)))
;;compile, then:
(setf bar (make-foo))
(setf (nth 0 (foo-a bar)) 4.5)
(foo-a bar)
-> (4.5 0.0)
(foo-b bar)
-> (4.5 0.0)

	I can understand doing constant folding on let-bound vars, but I'm
not so sure about struct slots.  The usual fix of changing it to:
(defstruct foo
	(a (list 0.0 0.0))
	(b (list 0.0 0.0)))
fixes the symptoms -- but I do think it's a bug.

	--chris sterritt