public static function resolve($key)
    {
        if ($key == 'B') return 0;
        $dict = static::$metric;
        if (preg_match(static::IEC_PATTERN, $key))
            $dict = static::$binary;
        if (array_key_exists($key, $dict)) return $dict[$key];
        throw new UnitNotFoundException(sprintf('Unit "%s" not found', $key));
    }