anchor.diff:  
   (defun make-immediate-input (value)
     (make-instance 'immediate-input
  +    :value value))
  +
  +(defclass external-input ()
  +  ((%value :initarg :value :reader value)))
  +
  +(defun make-external-input (value)
  +  (make-instance 'external-input
       :value value))
   
   (defclass instruction ()
  
positive.code:     (format stream "  ~a [shape = box, label = \"~a\"]~%" 
  	    (gethash info *instruction-table*)
  	    (sicl-env:name (sicl-env:location info)))
      (format stream "  ~a -> ~a [color = green]~%"
  	    (gethash info *instruction-table*)
  	    (gethash (sicl-env:location info) *instruction-table*))))
  
  
negative.code: (defmethod draw-location-info ((info immediate-input) stream)
    (when (null (gethash info *instruction-table*))
      (setf (gethash info *instruction-table*) (gensym))
      (format stream "   ~a [shape = ellipse, style = filled, fillcolor = green]~%"
  	    (gethash info *instruction-table*))
      (format stream "   ~a [label = \"~a\"]~%"
  	    (gethash info *instruction-table*)
  	    (value info))))
  
  (defmethod draw-location-info ((info external-input) stream)
    (when (null (gethash info *instruction-table*))
      (setf (gethash info *instruction-table*) (gensym))
      (format stream "   ~a [shape = ellipse, style = filled, fillcolor = pink]~%"
  	    (gethash info *instruction-table*))
      (format stream "   ~a [label = \"~a\"]~%"
  	    (gethash info *instruction-table*)
  	    (value info))))
  
  