public function setIncludeUpperBound($includeUpperBound)
    {
        if (is_bool($includeUpperBound)) {
            $this->includeUpperBound = $includeUpperBound;
        } else {
            $msg = "The includeUpperBound argument must be a boolean, '" . gettype($includeUpperBound) . "' given.";
            throw new InvalidArgumentException($msg);
        }
    }