Thus, to use categories for an image filtering system would indeed require constructing a category for the specific purpose of exclusion. Big ALA "actually, that *is* censorship" alarm goes off.
This is true, and I agree. but...
- The category system is constructed of minute subcategories, not
broad categories that are then combined.
You could then say "this and everything under it." But then you run into:
- The category system is not very consistent.
- The category system is not free of loops.
- An image on en:wp could be a local image (one system of categories)
or a Commons image (a completely different system of categories).
This is largely an engineering problem; and it can probably be overcome with some architecture work. As we are going to be implementing a major new feature *anyway* it's not something to reject outright, I think :)
Obviously given the complexity of the category tree system any such engineering wouldn't be infallible - but you could match it to most use cases. Ultimately it is just a collapsing tree problem, and they are ten a penny to a decent engineer :)
Tom