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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 MetaGraph(self):
 """Return the metagraph definition, if there is one. Raises: ValueError: If there is no metagraph for this run. Returns: The `meta_graph_def` proto. """

    if self._meta_graph is None:
      raise ValueError('There is no metagraph in this EventAccumulator')
    meta_graph = meta_graph_pb2.MetaGraphDef()
    meta_graph.ParseFromString(self._meta_graph)
    return meta_graph