protected function expToString($exp)
{
list(, $op, $left, $right, $inParens, $whiteLeft, $whiteRight) = $exp;
$content = array($this->reduce($left));
if ($whiteLeft) {
$content[] = " ";
}
$content[] = $op;
if ($whiteRight) {
$content[] = " ";
}
$content[] = $this->reduce($right);
return array("string", "", $content);
}