public function negotiate($request)
    {
        $media = $this->_classes['media'];
        foreach ($request->accepts() as $mime => $value) {
            if ($format = $media::suitable($request, $mime)) {
                $this->format($format);
                return;
            }
        }
        $mimes = join('", "', array_keys($request->accepts()));
        throw new NetException("Unsupported Media Type: [\"{$mimes}\"].", 415);
    }