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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 Graph(self):
 """Return the graph definition, if there is one. If the graph is stored directly, return that. If no graph is stored directly but a metagraph is stored containing a graph, return that. Raises: ValueError: If there is no graph for this run. Returns: The `graph_def` proto. """

    graph = graph_pb2.GraphDef()
    if self._graph is not None:
      graph.ParseFromString(self._graph)
      return graph
    raise ValueError('There is no graph in this EventAccumulator')