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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_graph_by_ids(self, network_ids: List[int]) -> BELGraph: """Get a combine BEL Graph from a list of network identifiers."""

        if len(network_ids) == 1:
            return self.get_graph_by_id(network_ids[0])

        log.debug('getting graph by identifiers: %s', network_ids)
        graphs = self.get_graphs_by_ids(network_ids)

        log.debug('getting union of graphs: %s', network_ids)
        rv = union(graphs)

        return rv