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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_entity_id(self, config, co_name):
 """ Use the CO name to construct the entity ID for the virtual IdP for the CO. The entity ID has the form {base_entity_id}/{co_name} :type config: satosa.satosa_config.SATOSAConfig :type co_name: str :rtype: satosa.satosa_config.SATOSAConfig :param config: satosa proxy config :param co_name: CO name :return: config with updated entity ID """

        base_entity_id = config['entityid']
        co_entity_id = "{}/{}".format(base_entity_id, quote_plus(co_name))
        config['entityid'] = co_entity_id

        return config