docs: def immediate_postdominators(self, end, target_graph=None):
          """
          Get all immediate postdominators of sub graph from given node upwards.
  
          :param str start: id of the node to navigate forwards from.
          :param networkx.classes.digraph.DiGraph target_graph: graph to analyse, default is self.graph.
  
          :return: each node of graph as index values, with element as respective node's immediate dominator.
          :rtype: dict
          """
          return self._immediate_dominators(end, target_graph=target_graph, reverse_graph=True)