In certain cases we iterator over the docids of the intermediate bool results (FBS) and for each doc we check if the docId is in at least one should clause. For certain filters we need to create a DocIdSetIterator, but I don't want to create this for each docId I need to check, so I create it first and reuse it for subsequent checks. I can do this because I iterate the docids in order in the intermediate result (FBS)