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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)