'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'

def immediate_postdominators(self, end, target_graph=None):

    return self._immediate_dominators(end, target_graph=target_graph, reverse_graph=True)
