Wonderful, many thanks for your very elaborate answer! Your solution looks much simpler than I had expected. I clearly recognize the architecture of CodeRunner as explained in the Readme on Github which had not been very clear to me before.
My strategy now is to walk through 'answer' by means of 're.sub()' with 'count=1' and creating a new instance of a simple class*, containing a single integer variable, for each respective loop variable. For efficiency, I shall substitute a while-loop in both cases. -- Does that sound reasonable or do you see any obvious simpler way?
As you correctly guessed, TigerJython cannot (easily) be called from a shell as it is an integrated GUI workbench written in Java running Jython. It has become quite popular in the German speaking parts of Europe for beginners courses in programming. Therefore I expect this template being of use to others as well. I shall report back when I have it working.
*) Edit: probably a list containing all the integer loop variables will do.