protected function expToString($exp)
{
list(, $op, $left, $right, , $whiteLeft, $whiteRight) = $exp;
$content = [$this->reduce($left)];
if ($whiteLeft) {
$content[] = ' ';
}
$content[] = $op;
if ($whiteRight) {
$content[] = ' ';
}
$content[] = $this->reduce($right);
return [Type::T_STRING, '', $content];
}