anchor.diff:  
   
   ;;;; Scratch ------------------------------------------------------------------
  -(with-zdd
  -  (enumerate (zdd-union (zdd-set '(1 2))
  -                        (zdd-set '(1 2))
  -                        (zdd-set '(1))
  -                        (zdd-family '(2) '(1 2) '(3)))))
  -
  -(with-zdd
  -  (enumerate (zdd-intersection (zdd-family '(1) '(1 2) '(3) )
  -                               (zdd-family '(2 3 4) '(1 4) nil '(1 2))
  -                               (zdd-family nil '(1 2)))))
  -
  -
  -(with-zdd
  -  (enumerate
  -    ))
  -
  -(with-zdd
  -  (enumerate
  -    (draw (zdd-meet (zdd-family '(1 2) '(1 6))
  -              (zdd-family '(2))))))
  -
  -
  -(with-zdd
  -  (-<> (zdd-join (zdd-family '(1 2) '(7 8) '())
  -                 (zdd-family '(1 5 9) nil)
  -                 (zdd-set '(1)))
  -    (print-enumerated <>)
  -    (zdd-keep-avoiders-of <> '(2 7))
  -    (print-enumerated <>)
  -    (draw <>)
  -    (zdd-size <>)
  -    )
  -  )
  +(let ((*draw-unique-sinks* nil))
  +  (with-zdd
  +    (-<> (zdd-join (zdd-family '(1 2) '(7 8) '())
  +                   (zdd-family '(1 5 9) nil)
  +                   (zdd-set '(1)))
  +      (print-enumerated <>)
  +      ; (zdd-keep-avoiders-of <> '(2 7))
  +      (print-enumerated <>)
  +      (draw <>)
  +      (zdd-size <>)
  +      )))
  
positive.code:     :attributes attributes))
  
  (defmethod cl-dot:graph-object-node ((graph (eql 'zdd))
                                       (object t))
    (make-instance 'cl-dot:node
      :attributes (ematch object
                    ((leaf c) `(:label ,(if c "⊤" "⊥") :shape :square))
                    ((node v) `(:label ,v :shape :circle)))))
  
  (defmethod cl-dot:graph-object-points-to ((graph (eql 'zdd))
                                            (object t))
    (ematch object
      ((leaf) '())
      ((node _ hi lo)
       (list (attrs hi :style :solid)
             (attrs lo :style :dashed)))))
  
  (defun draw (zdd &optional (filename "zdd.png"))
    (cl-dot:dot-graph
      (cl-dot:generate-graph-from-roots 'zdd (list zdd))
      filename
      :format :png)
    zdd)
  
negative.code: 
  ;;;; GraphViz -----------------------------------------------------------------
  (setf cl-dot:*dot-path* "/usr/local/bin/dot")
  (defparameter *draw-unique-sinks* nil)
  
  (defun attrs (object &rest attributes)
    (make-instance 'cl-dot:attributed
  