Hello,
Attention, please: This is a very old thread.
Today this can easily be done in XLSForm (Excel) with a constraint like:
if( selected(., ‘Z’) or selected(., ‘Y’), count-selected(.) = 1, true() ).
constraint_message
NONE (‘Z’) or Don’t know (‘Y’) can only be selected alone. Z and Y being the names of these choices.
You can also use the search function of this forum to find (recent) examples, like
https://community.kobotoolbox.org/t/how-to-add-multiple-constraints-none-and-declined-to-answer-for-multiple-choice-question/14779
