[TYPO3] How to use a different template for each RSS feed?

[Sergio Catalá Gil]

Recovering one of my past posts:

"I would like to use a different template for each RSS feed and each
category. I mean: when I access the feed for a certain category, I would
like only some fields are shown in the feed. I've seen you can control
this for the whole tt_news extension through
"plugin.tt_news.displayXML.rss2_tmplFile" but I would like to separate a
different template for each category". 

I got one idea to achieve my objective: only if the URL contains a
certain string, I would use another template. 

The idea to use this TS code came to me:

[globalVar = GP:tt_txnews['cat'] = 4]
plugin.tt_news.displayXML.rss2_tmplFile = EXT:tt_news/res/rss_2bis.tmpl

but it didn't work. 

Anyone would know how to know if the URL of my page contains a certain
parameter or string?

Thanks in advance, 
Sergio