public function openUrl(string $url)
    {
        if (!($url = $this->c::mbTrim($url))) {
            return; // Not possible.
        }
        $url_arg = escapeshellarg($url);

        if ($this->c::isMac()) {
            `open $url_arg`;
        } elseif ($this->c::isLinux()) {
            `xdg-open $url_arg`;
        } elseif ($this->c::isWindows()) {
            `start $url_arg`;
        } else {
            throw $this->c::issue('Unable to open <'.$url.'>. Unsupported operating system.');
        }
    }