anchor.diff:  (defmethod edge-source ((e edge))
     (edge-src e))
   
  +;;; This function returns the source state identification
  +;;; of this edge.
  +(defmethod edge-source ((e fst-edge))
  +  (fst-edge-src e))
  +
   ;;; This function returns the symbol of this edge.
  -(defmethod edge-symbol (e)
  -  (nth 1 e))
  +(defmethod edge-symbol ((e cons))
  +  (edge-symbol (build-edge e)))
   
   ;;; This function returns the symbol of this edge.
   (defmethod edge-symbol ((e edge))
  
positive.code: (defun build-edge (e)
    (if (equal (length e) 4)
        (make-fst-edge
         :src (edge-source e)
         :input-symb (edge-input e)
         :output-symb (edge-output e))
      (make-edge
       :src (edge-source e) 
       :symb (edge-symbol e) 
       :dst (edge-destination e))))
  
  ;;; This function returns the source state identification
  ;;; of this edge.
  (defmethod edge-source (e)
    (nth 0 e))
  
  ;;; This function returns the source state identification
  
negative.code: 	    (string (edge-symbol edge)))
  	(edge-destination edge)))
  
  