Another EQ problem

[Chris Lindblad <CJL@reagan.ai.mit.edu>]

    Date: 26 May 88 11:01 EST
    From:     STERRITT%SDEVAX.decnet@ge-crd.arpa

	    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!
	    E.G.:
    (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)

Never ever side effect constant structure, unless you want to be really faked
out.  Not only will (foo-b bar) be affected if you rplaca (foo-a bar), but
also (foo-a (make-foo)) will ALSO return what you smashed in (foo-a bar).
That's because you have smashed the list that is used to initialize the data
structure.