protected function expToString($exp)
    {
        list(, $op, $left, $right, /* $inParens */, $whiteLeft, $whiteRight) = $exp;

        $content = [$this->reduce($left)];

        if ($whiteLeft) {
            $content[] = ' ';
        }

        $content[] = $op;

        if ($whiteRight) {
            $content[] = ' ';
        }

        $content[] = $this->reduce($right);

        return [Type::T_STRING, '', $content];
    }