Thank, works now. Only problem left: method isoOptional was missing. To see this you have to click attempt.
Added it to my solution and it was accepted. You may add it to the template.

In isoEUTest you use methods conv, lrl and rlr. There is no implementation for these methods. I even can't find a signature. I checked the code for other languages but found nothing useful.
As expected my solution fails as test classes can't compile.