docs: public IndexDoc getDoc(String field, int docId) {
      if (fieldReferences.containsKey(field)) {
        FieldReferences fr = fieldReferences.get(field);
        try {
          IndexInput inIndexDocId = indexInputList.get("indexDocId");
          ArrayList<MtasTreeHit<?>> list = CodecSearchTree.searchMtasTree(docId,
              inIndexDocId, fr.refIndexDocId, fr.refIndexDoc);
          if (list.size() == 1) {
            return new IndexDoc(list.get(0).ref);
          }
        } catch (IOException e) {
          log.debug(e);
          return null;
        }
      }
      return null;
    }