anchor.diff:  	  (*datum-table* (make-hash-table :test #'eq)))
         (format stream "digraph G {~%")
         (format stream "   start [label = \"START\"];~%")
  -      (draw-instruction start stream)
  +      ;; Assign a unique ID to each instruction and each datum.
  +      (map-instructions-arbitrary-order
  +       (lambda (instruction)
  +	 (when (null (gethash instruction *instruction-table*))
  +	   (setf (gethash instruction *instruction-table*) (gensym))
  +	   (loop for datum in (append (inputs instruction) (inputs instruction))
  +		 when (null (gethash datum *datum-table*))
  +		   do (setf (gethash datum *datum-table*) (gensym)))))
  +       initial-instruction)
  +      ;; Draw all instructions together with inputs and outputs.
  +      (map-instructions-arbitrary-order
  +       (lambda (instruction)
  +	 (draw-instruction instruction stream))
  +       initial-instruction)
  +      ;; Draw a START label to indentify the initial instruction.
         (format stream "start -> ~a [style = bold];~%"
  -	      (instruction-id start))
  +	      (instruction-id initial-instruction))
         (format stream "}~%"))))
   
   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  
positive.code: 		   (datum-id datum)
  		   (instruction-id instruction)
  		   i))
    (loop for datum in (outputs instruction)
  	for i from 1
  	do (draw-datum datum stream)
  
negative.code:   (gethash instruction *instruction-table*))
  
  (defgeneric draw-instruction (instruction stream))
  
  (defmethod draw-instruction :around (instruction stream)
    (format stream "  ~a [shape = box];~%"
  	  (instruction-id instruction))
    ;; Draw a numbered bold arrow to each successor.
    (loop for next in (successors instruction)
  	for i from 1
  	do (format stream
  		   "  ~a -> ~a [style = bold, label = \"~d\"];~%"
  		   (instruction-id instruction)
  		   (instruction-id next)
  		   i))
    ;; Draw a numbered red dashed arrow from each input.
    (loop for datum in (inputs instruction)
  	for i from 1
  	do (draw-datum datum stream)
  