Thread:Extension talk:ParserFunctions/Using expr - Function in inline queries

I am trying to do something like this:

while MyTemplate does nothing more than

+

So I try to ask for all articles that fit the conditions above. What I get as a result is a number, no text, thats defined by convention. And each result is put in the template, adding a "+". The outer-Expression #expr should receive these results and add them. But all I get is an Error, saying that the char "[" is undefinded. I guess the inner-ask is responsible for this message because of the conditions Category:MyCategory etc.

Is there a way of solving this problem or maybe there are alternatives fitting my purposes?