public function assertGreaterThan($config, $keyGreater, $keyLower)
    {
        if (!isset($config[$keyLower])
            || !isset($config[$keyGreater])
            || $config[$keyLower] >= $config[$keyGreater]) {
            throw new ConfigurationException($keyGreater." must be greater than ".$keyLower);
        }
        return $this;
    }