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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_backward_star(self, node):
 """Given a node, get a copy of that node's backward star. :param node: node to retrieve the backward-star of. :returns: set -- set of hyperedge_ids for the hyperedges in the node's backward star. :raises: ValueError -- No such node exists. """

        if node not in self._node_attributes:
            raise ValueError("No such node exists.")
        return self._backward_star[node].copy()