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