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:   "Return all cycles starting and ending in A."
    (mapcan (lambda (edge &aux (n (end-node edge)))
              (cons edge (paths graph n a)))
            (edges graph a)))
  
  ;;; p83
  
  
negative.code:               (if (eq n b)
                    (list edge)
                    (cons edge (paths graph n b (cons a visited))))))
            (edges a graph)))
  
  ;;; p82
  
  