>>Exclude words may be a good option, but I'm afraid it would be harder to implement than new type of search (Only threads started). However, I don't know, may be both of these options are quite hard to implement based on the current database structure.
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>I'd say excluded words should be pretty simple to do. Instead of "for xxx $ yyy", "for !(xxx $ yyy)" or instead of "where xxx in (yyy)", "where xxx not in (yyy)".

I don't know for sure, but I believe Michel used PHDBase library for the search on this site. That's why I'm not sure how easy is to implement exclude option.