protected function compare($a, $b)
    {
        switch ($this->operator) {
            case '=':
            case '==': return $a == $b;
            case '!=': return $a != $b;
            case '<':  return $a < $b;
            case '>':  return $a > $b;
            case '<=': return $a <= $b;
            case '>=': return $a >= $b;
        }
    }