Custom WordPress Feeds operators?

I wish to create a feed of my blog which is a feed of two categories OR the feed of some tag and not the feed of some other category

I know how to do "have a feed of this1 + this2 categories", but how do you do "OR" in a feed URL? (and in general, is more complex operators on the URL possible, and how?)

Links that don't answer the question (but help define it):

• http://lorelle.wordpress.com/2006/03/27/customizing-rss-feed-links-for-wordpresscom-and-wordpress-sidebar-widgets/
• http://dailycupoftech.com/2007/07/25/creating-custom-wordpress-feeds/