public abstract boolean compare(A actual, E expected);
@Test
  public void testFormattingDiffsUsing_compare() {
    // The compare behaviour should be the same as the wrapped correspondence.
    assertThat(LENGTHS_WITH_DIFF.compare("foo", 3)).isTrue();
    assertThat(LENGTHS_WITH_DIFF.compare("foot", 4)).isTrue();
    assertThat(LENGTHS_WITH_DIFF.compare("foo", 4)).isFalse();
  }