public function setUpperBound($upperBound)
    {
        if (is_float($upperBound) === true) {
            $this->upperBound = $upperBound;
        } else {
            $msg = "The 'upperBound' argument must be a float value, '" . gettype($upperBound) . "' given.";
            throw new InvalidArgumentException($msg);
        }
    }