Richer RSS selection expressions with negation

I've added the not operation to my feed algebra, which can be used to exclude entries matching a given tag from the final RSS feed, e.g.

http://eigenclass.org/hiki.rb/c=rss;tags=-subpar+blog

would return entries tagged as blog but not as subpar (I'll probably use the latter for postings such as this one). It is thus possible to construct personalized feeds according to the expression

where represents all the nodes on eigenclass.org and is the selection operation as in relational algebra.

To sum up:

http://eigenclass.org/hiki.rb?c=rss;tags=a+b+c+-d

entries matching a, b and c but not d

http://eigenclass.org/hiki.rb?c=rss;tags=a,b,c

posts tagged as a or (inclusive) b or c.

http://eigenclass.org/hiki.rb?c=rss;tags=a+b+-c,c+d,e

articles matching (a AND b AND NOT(c)) or (c AND d) or e

Orthogonally to the fairly rich selection expressions, there are several differential representations in addition to the default full-text one.

Short of generalizing selection expressions to allow arbitrary operation ordering, my RSS plugin is about as complete as it can get.