anchor.diff:  
   (defmethod stream-draw-ast ((ast block-ast) stream)
     (format stream "   ~a [label = \"block\"];~%"
  -	  (id ast))
  -  (stream-draw-ast (body-ast ast) stream)
  -  (format stream "   ~a -> ~a~%"
  -	  (id ast)
  -	  (id (body-ast ast))))
  +	  (id ast)))
     
   (defmethod body-ast ((ast block-ast))
     (first (children ast)))
  
positive.code: 
  (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))))
  
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  
negative.code: 
  (defmethod stream-draw-ast ((ast call-ast) stream)
    (format stream "   ~a [label = \"call\"];~%"
  	  (id ast)))
  
  (defmethod callee-ast ((ast call-ast))
    (first (children ast)))
  