there are many ways of doing this and I think we should keep it simple. Simple here means not too many options ie. settle on one implemenation. What I really like about the max is that it is a true lowerbound since it says the term occurs in at least max(DF) documents. If you want to be absolutely correct use a Similarity that uses `TTF` instead. We can make it easy to exchange that though but I think most of the users won't see any difference here? It's really hard to say what makes more sense here. I will put it in as a TODO for now ok?