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