anchor.diff:      fsa))
   
   (defun fsa-remove-edge! (fsa src-label input-symbol dst-label)
  -  (let ((src-node (hash-table-ref/default (fsa-builder-nodes fsa) src-label #f))
  -	(dst-node (hash-table-ref/default (fsa-builder-nodes fsa) dst-label #f)))
  +  (let ((src-node (hash-table-ref/default (fsa-builder-nodes fsa) src-label nil))
  +	(dst-node (hash-table-ref/default (fsa-builder-nodes fsa) dst-label nil)))
       (if (and src-node dst-node)
   	(node-remove-edge! src-node input-symbol dst-node))
       fsa))
  
positive.code: (defun graphviz-export-to-file (fsa file) 
    "This function will write the dot description of the FSA in the stream."
    (let ((p (open-output-file file)))
      (display (format "digraph G {~%  rankdir = LR;~%  size = \"8, 10\";~%") 
  	     p)
      (if (not (null? (fsa-builder-finals fsa)))
  	(progn
  	  (display (format "~%  node (shape = doublecircle);~% ")
  		   p)
  	  (map (lambda (x) 
  		 (display (format " \"~A\"" (node-label x))
  			  p))
  	       (fsa-builder-finals fsa))
  	  (display ";")))
      (display (format "~%~%  node (shape = circle);~% ")
  	     p)
      (map (lambda (label)
  	   (display (format " \"~A\"" label)
  		    p))
  	 (hash-table-keys (fsa-builder-nodes fsa)))
      (display (format ";~%~%")
  	     p)
      (map (lambda (node)
  	   (map (lambda (edge)
  		  (display (format "  \"~A\" -> \"~A\" (label = \"~A\");~%"
  				   (car edge)
  				   (caddr edge)
  				   (if (null? (cadr edge))
  				       "epsilon"
  				     (cadr edge)))
  			   p))
  		(node-edges node)))
  	 (hash-table-values (fsa-builder-nodes fsa)))
      (display (format "}~%") 
  	     p)
      (close-output-port p)
      fsa))
  
  
negative.code: 				   (setf nodes (cons (car n) nodes))
  				   (retreive-nodes (append (cdr n) (lset-difference eq
  										    (node-destinations (car n))
  										    nodes))))))
  	       (build-fsa-builder-with-nodes
  		(lambda ()
  		  (for-each (lambda (node)
  			      (fsa-add-node! fsa-builder node))
  			    nodes))))
  	      (retreive-nodes nodes))
        fsa-builder))
  
  