(ql:quickload :place-utils)

(define-setf-expander iff (cond then-place else-place &environment env)
  (multiple-value-bind (temps1 exprs1 stores1 set1 get1)
      (get-setf-expansion then-place env)
    (multiple-value-bind (temps2 exprs2 stores2 set2 get2)
        (get-setf-expansion else-place env)
      (values (append temps1 temps2)
              (append (mapcar (lambda (e)
                                `(when ,cond ,e))
                              exprs1)
                      (mapcar (lambda (e)
                                `(unless ,cond ,e))
                              exprs2))
              (append stores1 stores2)
              `(if ,cond ,set1 ,set2)
              `(if ,cond ,get1 ,get2)))))

(let ((a (cons 1 2))
      (val nil))
  (place-utils:readf (iff val (car a) (cdr a))))