anchor.diff:    "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)))
  +          (edges a graph)))
   
   ;;; p83
   
  
positive.code:   (munion (lambda (node 
                     &aux (edges (remove-if (lambda (edge)
                                              (subsetp (nodes edge) visited-ns))
                                    (sort #'edge< (set-difference (edges graph node) visited-es)))))
               ;; only pick the edge with the least weight
               (when edges
                  (mapcar (lambda (tree) (cons (first edges) tree))
  
negative.code:                           (mapcar (lambda (tree) (cons edge tree))
                              (s-trees graph (cons node visited-ns)
                                             (cons edge visited-es)))))
                 (set-difference (edges node graph) visited-es)))
            (set-difference (graph-nodes graph) visited-ns)))
  
  (defun s-tree-p (graph tree)
  