text: """An example of persistence for a directed graph structure.   The
  graph is stored as a collection of edges, each referencing both a
  "lower" and an "upper" node in a table of nodes.  Basic persistence
  and querying for lower- and upper- neighbors are illustrated::
  
      n2 = Node(2)
      n5 = Node(5)
      n2.add_neighbor(n5)
      print n2.higher_neighbors()
  
  .. autosource::
  
  """