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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 get_buildenv_graph(self):
 """Return a graph induced by buildenv nodes"""

        # This implementation first obtains all subsets of nodes that all
        # buildenvs depend on, and then builds a subgraph induced by the union
        # of these subsets. This can be very non-optimal.
        # TODO(itamar): Reimplement efficient algo, or redesign buildenvs
        buildenvs = set(target.buildenv for target in self.targets.values()
                        if target.buildenv)
        return nx.DiGraph(self.target_graph.subgraph(reduce(
            lambda x, y: x | set(y),
            (get_descendants(self.target_graph, buildenv)
             for buildenv in buildenvs), buildenvs)))