public @Nullable String formatDiff(A actual, E expected) {
    return null;
  }
@Test
  public void testFormattingDiffsUsing_formatDiff() {
    assertThat(LENGTHS_WITH_DIFF.formatDiff("foo", 4)).isEqualTo("-1");
    assertThat(LENGTHS_WITH_DIFF.formatDiff("foot", 3)).isEqualTo("1");
  }