MockDirectoryWrapper dir = newMockDirectory();
dir.setAssertNoUnrefencedFilesOnClose(true);
IndexWriter iw = new IndexWriter(dir, new IndexWriterConfig(TEST_VERSION_CURRENT, null));
iw.addDocument(new Document());
iw.close();
IndexOutput output = dir.createOutput("_hello.world", IOContext.DEFAULT);
output.writeString("i am unreferenced!");
output.close();
dir.sync(Collections.singleton("_hello.world"));
dir.close();
}