protected function lookup($system)
    {
        foreach (static::$conversionTable as $row) {
            if (isset($row[$this->system], $row[$system]) && $row[$this->system] === $this->value) {
                return $row[$system];
            }
        }
        throw new \Exception(sprintf(
            'Failed to look up value "%s" from system "%s" to system "%s"',
            $this->value,
            $this->system,
            $system
        ));
    }