At the moment I got the PopUp s to work by using very bland strings. No leading blanks, nothing like a^{-1} or $SPACE or \( or \cdot. I guess it's good enough for my purposes. If the strings
in the call
$apop1=PopUp(['?',$arX1,$arX2,$arX3,$arX4,$arX5,$none],$arX1);
are not bland the messages column that appears when the "Check answers" button is clicked will contain something like
Your answer should be one of By A=A+0 with A=b, ?, By 0+A=A with A=-a, By A+(-A)=0 with A=a, By A+(B+C)=(A+B)+C with A=b , B=a , C=-a, By the hypothesis that b+a=0 or None of the above
The strings that appear in this message are the values of the variables $arX1 ... $arX5, $none. I got the message to appear by putting a leading blank in one of the strings.