Rational Subsets of HNN extensions :O :O

The hero found that there is gonna be a talk by some prof from some unknown university. So he inquired about it. Our hero's prof also told to attend the talk saying that the prof is working in good things of 'formal languages'. So the hero got pretty interested and attended the talk early in the morning at 9.30. Hero sat among the most elite ppl in the informatik department and the talk started with an intro in German :O .. Hero couldn't understand a bit and then things just became comfortable with english slides and english talk starting. But right from the first line everything turned highly incomprehensible and the hero suffered for full 1hr and 15 mins. listening the the most enlightening talk which was to be shown in some conference. So now the hero has to beware of the most technical and mad ppl here whose prime intention is to go terribly deep in something and pain the innocent souls.