public void testMergeSorted_stable_allEqual() {
    ImmutableList<TestDatum> first = ImmutableList.of(new TestDatum("A", 1), new TestDatum("A", 2));
    ImmutableList<TestDatum> second =
        ImmutableList.of(new TestDatum("A", 3), new TestDatum("A", 4));

    Comparator<TestDatum> comparator = Comparator.comparing(d -> d.letter);
    Iterator<TestDatum> merged =