Actually shouldn't that be NOT IsDefined("...") AND NOT
IsDefined("...")
Perhaps. I did add "depending on what you want", which leaves
some leeway. The key suggestion to JohnGree was that the
isDefined() function has to be repeated for every variable. Hence,
isDefined("var_A") and/or isDefined("var_B"), rather than
isDefined("var_A") and/or ("var_B").