public function toArbitraryBase(string $alphabet) : string
    {
        $base = \strlen($alphabet);

        if ($base < 2) {
            throw new \InvalidArgumentException('The alphabet must contain at least 2 chars.');
        }

        if ($this->value[0] === '-') {
            throw new NegativeNumberException(__FUNCTION__ . '() does not support negative numbers.');
        }

        return Calculator::get()->toArbitraryBase($this->value, $alphabet, $base);
    }