code: def nodes_by_betweenness(self, treshold=0.0):
          
          nodes = [(n.betweenness, n) for n in self.nodes if n.betweenness > treshold]
          nodes.sort(); nodes.reverse()
          return [n for w, n in nodes]
docs: Returns nodes sorted by betweenness centrality.
          Nodes with a lot of passing traffic will be at the front of the list.