Markdown Extra doesn't work

Hi,

thanks for the basic information.
If I use it like written in the readme.md I get an infinite loop. If I add the command:

require 'vendor/autoload.php';

Because the file loads itself again, again and again …

File: vendor/autoload.php

<?php

// autoload.php @generated by Composer

require_once __DIR__ . '/composer/autoload_real.php';
// require 'vendor/autoload.php   //not good idea to do that

return ComposerAutoloaderInit6cc5f2042b8b4366e64fccdea6a288b9::getLoader();

Now I was trying again to use this answer Link

But still, my textarea looks like this:

What do I miss? Suggestions?

best regards
ET