Wow. Thanks, Hilary. After I posted the question I continued to work on it for several more hours and finally cobbled together this formula:
contains(xdMath:Eval(xdMath:Eval(../my:ProductGroup/my:Item, 'concat(my:Reason, ";")'), ".."), "Other")
Your way is much easier. I never noticed that dropdown list of "Any occurrence of..." etc existed!
Thanks again!!