docs: public IndexDoc getNextDoc(String field, int previousDocId) {
      if (fieldReferences.containsKey(field)) {
        FieldReferences fr = fieldReferences.get(field);
        try {
          if (previousDocId < 0) {
            return new IndexDoc(fr.refIndexDoc);
          } else {
            int nextDocId = previousDocId + 1;
            IndexInput inIndexDocId = indexInputList.get("indexDocId");
            ArrayList<MtasTreeHit<?>> list = CodecSearchTree.advanceMtasTree(
                nextDocId, inIndexDocId, fr.refIndexDocId, fr.refIndexDoc);
            if (list.size() == 1) {
              IndexInput inDoc = indexInputList.get("doc");
              inDoc.seek(list.get(0).ref);
              return new IndexDoc(inDoc.getFilePointer());
            }
          }
        } catch (IOException e) {
          log.debug(e);
          return null;
        }
      }
      return null;
    }