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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_ipv4_table(self):
 """Returns global IPv4 table. Creates the table if it does not exist. """


        vpn_table = self._global_tables.get(RF_IPv4_UC)
        # Lazy initialize the table.
        if not vpn_table:
            vpn_table = Ipv4Table(self._core_service, self._signal_bus)
            self._global_tables[RF_IPv4_UC] = vpn_table
            self._tables[(None, RF_IPv4_UC)] = vpn_table

        return vpn_table