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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 _add_redundancy_routers(self, context, start_index, stop_index, user_visible_router, ports=None, ha_settings_db=None, create_ha_group=True):
 """Creates a redundancy router and its interfaces on the specified subnets. """

        priority = (DEFAULT_MASTER_PRIORITY +
                    (start_index - 1) * PRIORITY_INCREASE_STEP)
        r = copy.deepcopy(user_visible_router)
        # No tenant_id so redundancy routers are hidden from user
        r['tenant_id'] = ''
        name = r['name']
        redundancy_r_ids = []
        for i in range(start_index, stop_index):
            del r['id']
            # We don't replicate the user visible router's routes, instead
            # they are populated to redundancy routers for get router(s) ops
            r.pop('routes', None)
            # Redundancy routers will never have a route spec themselves
            # The redundancy routers must have HA disabled
            r[ha.ENABLED] = False
            r['name'] = name + REDUNDANCY_ROUTER_SUFFIX + str(i)
            # set role so that purpose of this router can be easily determined
            r[routerrole.ROUTER_ROLE_ATTR] = ROUTER_ROLE_HA_REDUNDANCY
            gw_info = r[EXTERNAL_GW_INFO]
            if gw_info and gw_info['external_fixed_ips']:
                # Ensure ip addresses are not specified as they cannot be
                # same as visible router's ip addresses.
                for e_fixed_ip in gw_info['external_fixed_ips']:
                    e_fixed_ip.pop('ip_address', None)
            r = self.create_router(context, {'router': r})
            LOG.debug("Created redundancy router %(index)d with router id "
                      "%(r_id)s", {'index': i, 'r_id': r['id']})
            priority += PRIORITY_INCREASE_STEP
            r_b_b = RouterRedundancyBinding(
                redundancy_router_id=r['id'],
                priority=priority,
                user_router_id=user_visible_router['id'])
            context.session.add(r_b_b)
            redundancy_r_ids.append(r['id'])
        for port_db in ports or []:
            port = self._core_plugin._make_port_dict(port_db)
            self._add_redundancy_router_interfaces(
                context, user_visible_router, None, port,
                redundancy_r_ids, ha_settings_db, create_ha_group)