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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 check_config(self, config):
 """ Check the config file for required fields and validity. @param config: The config dict. @return: True if valid, error string if invalid paramaters where encountered. """

        validation = ""
        required = ["name", "currency", "IBAN", "BIC"]

        for config_item in required:
            if config_item not in config:
                validation += config_item.upper() + "_MISSING "

        if not validation:
            return True
        else:
            raise Exception("Config file did not validate. " + validation)