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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_fail_graph(self, failure_index=None):
 """Returns a graph showing a solve failure. Args: failure_index: See `failure_reason` Returns: A pygraph.digraph object. """

        phase, _ = self._get_failed_phase(failure_index)
        return phase.get_graph()