code: private IndexInput openMtasFile(SegmentReadState state, String name,
        String extension) throws IOException {
      return openMtasFile(state, name, extension, null, null);
    }
docs: Open mtas file.
  
  @param state the state
  @param name the name
  @param extension the extension
  @return the index input
  @throws IOException Signals that an I/O exception has occurred.