anchor.diff:                            (mapcar (lambda (tree) (cons edge tree))
                               (s-trees graph (cons node visited-ns)
                                              (cons edge visited-es)))))
  -               (set-difference (edges graph node) visited-es)))
  +               (set-difference (edges node graph) visited-es)))
             (set-difference (graph-nodes graph) visited-ns)))
   
   (defun s-tree-p (graph tree)
  
positive.code:               (if (eq n b)
                    (list edge)
                    (cons edge (paths graph n b (cons a visited))))))
            (edges graph a)))
  
  ;;; p82
  
  
negative.code:   (munion (lambda (node 
                     &aux (edges (remove-if (lambda (edge)
                                              (subsetp (nodes edge) visited-ns))
                                    (sort #'edge< (set-difference (edges node graph) visited-es)))))
               ;; only pick the edge with the least weight
               (when edges
                  (mapcar (lambda (tree) (cons (first edges) tree))
  