public function assertInteger($config, $key, $lowest = null, $highest = null)
    {
        if (array_key_exists($key, $config)) {
            if (!is_int($config[$key])) {
                throw new ConfigurationException($key." must be an integer.");
            }
            if ($lowest !== null && $config[$key] < $lowest) {
                throw new ConfigurationException($key." cannot be lower than ".$lowest);
            }
            if ($highest !== null && $config[$key] > $highest) {
                throw new ConfigurationException($key." cannot be higher than ".$highest);
            }
        }
        return $this;
    }