(defun custom-proper-cons (a b)
  "Return a proper cons of A and B, but avoid consing NIL."
  (etypecase a
    (null (etypecase b
	    (null nil)
	    (atom (list b))
	    (list b)))
    (atom (etypecase b
	    (null (list a))
	    (atom (list a b))
	    (list (cons a b))))
    (list (etypecase b
	    (null a)
	    (atom (cons a (list b)))
	    (list (cons a b))))))