anchor.diff:  
   (defmethod stream-draw-ast ((ast s<=-ast) stream)
     (format stream "   ~a [label = \"s<=\"];~%"
  -	  (id ast))
  -  (loop for argument-ast in (argument-asts ast)
  -	do (stream-draw-ast argument-ast stream)
  -	   (format stream "   ~a -> ~a~%" (id ast) (id argument-ast))))
  +	  (id ast)))
   
   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
   ;;;
  
positive.code: 
  (defmethod stream-draw-ast ((ast load-car-ast) stream)
    (format stream "   ~a [label = \"load-car\"];~%"
  	  (id ast))
    (stream-draw-ast (form-ast ast) stream)
    (format stream "   ~a -> ~a~%"
  	  (id ast)
  	  (id (form-ast ast))))
  
  (defmethod form-ast ((ast load-car-ast))
    (first (children ast)))
  
negative.code: 
  (defmethod stream-draw-ast ((ast load-class-ast) stream)
    (format stream "   ~a [label = \"load-class\"];~%"
  	  (id ast)))
  
  (defmethod form-ast ((ast load-class-ast))
    (first (children ast)))
  