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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 as_graph_queue(self, manifest, limit_to=None):
 """Returns a queue over nodes in the graph that tracks progress of dependecies. """

        if limit_to is None:
            graph_nodes = self.graph.nodes()
        else:
            graph_nodes = limit_to

        new_graph = _subset_graph(self.graph, graph_nodes)
        return GraphQueue(new_graph, manifest)