public function setSpan($span)
    {
        if (is_int($span) === true && $span > 0) {
            $this->span = $span;
        } else {
            $msg = "The 'span' attribute must be a strictly positive (> 0) integer, '" . gettype($span) . "' given.";
            throw new InvalidArgumentException($msg);
        }
    }