new DoubletonIteratorTester() {
      @Override
      protected Iterator<Integer> newTargetIterator() {
        return Iterators.concat(iterateOver(1), iterateOver(2));
      }
    }.test();
  }

  @GwtIncompatible // fairly slow (~40s)
  public void testConcatSingletonAndSingletonWithEmptiesYieldsDoubleton() {
    new DoubletonIteratorTester() {
      @Override