NOT's in "ul" or "t" params?

[Damon Tkoch]

Hello, Is there a way to use a negating value in "ul" or "t" search parms?   For example, search for "/directory_a/", "/directory_b/", or "/directory_c", or "NOT directory_a".   Also, what is the state of the categories support?  Categories.txt claims alpha code.  How stable is this?   -Damon