protected function getTransposeClosure($operator, $value)
    {
        $value = hexdec($value) * ($operator === '-' ? -1 : 1);
        $converter = $this->converter;

        return function ($matches) use ($value, $converter) {
            return $converter->toUTF8Character(
                $converter->toUnicodeCodepoint($matches[0]) + $value
            );
        };
    }