Really kidding, now

[Guy.Steele@CMU-CS-A]

(defsetf logand (x y) (newval)
  `(if (= (logand y newval) newval)
       (progn (setf ,x (logior newval
			       (logand (random (expt 2 (max (integer-length
							       ,newval)
							    (integer-length
							       ,y))))
				       (lognot y))))
	      ,newval)
       (error "Impossible setf.")))

So then when you say

(setf (logand x 12) 8)

x will get set to some random value such that when it is anded with
12 you will get 8.  (X might get set to 9 or 10, for example.)
--Guy