code: public File persist(
              final IncrementalIndex index,
              final Interval dataInterval,
              File outDir,
              IndexSpec indexSpec
      ) throws IOException {
          return persist(index, dataInterval, outDir, indexSpec, new BaseProgressIndicator());
      }
docs: This is *not* thread-safe and havok will ensue if this is called and writes are still occurring
  on the IncrementalIndex object.
  
  @param index        the IncrementalIndex to persist
  @param dataInterval the Interval that the data represents
  @param outDir       the directory to persist the data to
  @return the index output directory
  @throws java.io.IOException if an IO error occurs persisting the index