tokens: Intuitively , it seems like this might be co-NP hard , since it 's easy to refute an answer to an optimization problem by guessing a better solution and using it as a witness , but I have no idea how to show this .
ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O