thanks to S68’s tut on lightning bolts I finally decided to enter the world of python. my first steps are not very successful though .
I installed latest python version, defined python path in blender, opened Lsystem tree-generator and ran the script by hitting alt-p. buttons occur the way they should. entered S68’s parameters, hit button “Generate” and here the problem occurs: nothing happens 'cept generator buttons dissapear and the raw script shows up again. I’m guessing I missed some important step, right?