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In CLOS, the way to access slots as if they were lexical variables is to
use WITH-SLOTS, which uses SYMBOL-MACROLET to macroexpand occurrences of
the slot names into (SLOT-VALUE <object> '<name>) forms.  It also
converts any SETQ special forms into SETF forms.

But what about MULTIPLE-VALUE-SETQ?  If SETQ is allowed to assign slot
variables then so should MULTIPLE-VALUE-SETQ.  At first I thought this
would require a MULTIPLE-VALUE-SETF, and perhaps that is the right way
to go anyway.  However, it could also be done by expanding the
requirement on SYMBOL-MACROLET: it could transform

(multiple-value-setq (<slot> ...) ...)


(progn (multiple-value-setq (#:GENSYM ...) ...)
       (setf (slot-value <object> '<slot>) #:GENSYM))

This works properly in Symbolics Flavors because

(multiple-value-setq (var1 var2) <form>)

macroexpands into

(multiple-value-call #'(lambda (#:g1 #:g2) (setq var1 #:g1 var2 #:g2)) <form>)

which reduces it to a solved problem.  Perhaps the CLOS committee was
assuming that MULTIPLE-VALUE-SETQ includes SETQ's in its expansion, but
this is not actually specified by CLtL.