In particular, I like
"
Alternatively, you might want to use the contextString.pl context, which allows only string answers, and only the ones your define. Here is one approach:
loadMacros("contextString.pl");
Context("String");
Context()->operators->redefine(',', using=>','); # allow lists of strings
Context()->strings->add(
"BC" => {caseSensitive=>1}, "CB" => {alias => "BC", caseSensitive=>1},
"AD" => {caseSensitive=>1}, "DA" => {alias => "AD", caseSensitive=>1},
);
BEGIN_PGML
[________]{"BC,AD"}
END_PGML
"