Yes, of course, thank you very much! My mistake was to think, that everything could be 'driven' from the startup code. I see now that this is neither possible nor in fact necessary when using TA both to set up the variables for the MathApp and to give the precise and individual feedback.
Ok, thank you. The possibility of allowing the student to navigate through other sites or programs while doing the exam is in discussion since it can be considered not compliant with standards concerning safe exams procedures, but this option can be taken into account.
Yes, I'm using the 2016 version. It worked, thank you! I used $x*$response.provachain1.1.1 in the response area, where $x=4; was defined as the only line of the algorithm. Good to know also that it is wrong to consider the (addable by the user) field "id" from "Information Fields" as equivalent to the "id" appearing in the source, which is not the case.