anchor.diff:  
   (defgeneric element-member-p (set element)
     (:documentation "Return T iff the element is in the set.")
  -  (:method (set element) (funcall (element-member-fn set) element)))
  +  (:method (set element) (funcall (element-member-fn set) set element)))
   
   
   ;;; Interval Class definitions
  
positive.code: ;; isn't standard use of an interval and could be an error. On the
  ;; other hand, I do rely on this behavior to easily do interval
  ;; operations. But it will break code if we try to define intervals on
  ;; circular sets.
  (defmethod make-interval :around (interval lower upper &rest args)
    (declare (ignore args interval))
    (if (< upper lower)
  
negative.code:                         :initarg :previous-element-fn)
     (element-member-fn :accessor element-member-fn
                        :initarg :element-member-fn))
    (:documentation "A list that cannot conain doublettes. The order is
  given by the property of the objects, not by the order of insertion
  into the set."))
  
  (defgeneric next-element-in-set (set element)
    (:documentation "The smallest element x in set, for which element < x.")
  