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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 _notify_subnet_create(resource, event, trigger, **kwargs):
 """Called when a new subnet is created in the external network"""

    context = kwargs['context']
    subnet = kwargs['subnet']
    l3plugin = bc.get_plugin(L3_ROUTER_NAT)
    for router in l3plugin.get_routers(context):
        if (router['external_gateway_info'] and
                (router['external_gateway_info']['network_id'] ==
                 subnet['network_id'])):
            router_data = {'router': router}
            l3plugin.update_router(context, router['id'], router_data)