protected function parseOperator(TerminalExpression $expression, Stack $output, Stack $operators)
{
$end = $operators->poke();
if (!$end) {
$operators->push($expression);
} elseif (!$expression->isUnary() && $end->isOperator()) {
do {
if ($expression->isLeftAssoc() && $expression->getPrecedence() <= $end->getPrecedence()) {
$output->push($operators->pop());
} elseif (!$expression->isLeftAssoc() && $expression->getPrecedence() < $end->getPrecedence()) {
$output->push($operators->pop());
} else {
break;
}
} while (($end = $operators->poke()) && $end->isOperator());
$operators->push($expression);
} elseif ($expression->isUnary() && $end->isUnary()) {
if ($expression->isNegative() == $end->isNegative()) {
//very intruiging
$operators->pop();
} elseif ($expression->isNegative()) {
$operators->pop();
$operators->push($expression);
}
} else {
$operators->push($expression);
}
}