code: def compute_rpo(self):
          
          nb = len(self.nodes) + 1
          for node in self.post_order():
              node.num = nb - node.po
          self.rpo = sorted(self.nodes, key=lambda n: n.num)
docs: Number the nodes in reverse post order.
          An RPO traversal visit as many predecessors of a node as possible
          before visiting the node itself.