docs: private String addIndexInputToList(String name, IndexInput in,
        String postingsFormatName) throws IOException {
      if (indexInputList.get(name) != null) {
        indexInputList.get(name).close();
      }
      if (in != null) {
        String localPostingsFormatName = postingsFormatName;
        if (localPostingsFormatName == null) {
          localPostingsFormatName = in.readString();
        } else if (!in.readString().equals(localPostingsFormatName)) {
          throw new IOException("delegate codec " + name + " doesn't equal "
              + localPostingsFormatName);
        }
        indexInputList.put(name, in);
        indexInputOffsetList.put(name, in.getFilePointer());
        return localPostingsFormatName;
      } else {
        log.debug("no " + name + " registered");
        return null;
      }
    }