public function subtract($left, $right) {
        if (empty($left)) {
            return $right;
        } elseif (empty($right)) {
            return $left;
        } elseif ($right[0] == '-') {
            return $this->add($left, substr($right, 1));
        } elseif ($left[0] == '-') {
            return '-' . $this->add(ltrim($left, '-'), $right);
        }
        $left    = $this->normalize($left);
        $right   = $this->normalize($right);
        $results = $this->subtractBinary($left, $right);
        $result  = BaseConverter::convertFromBinary($results[1], '0123456789');
        return $results[0] . $result;
    }