code: @NullableDecl
    public String formatDiff(@NullableDecl A actual, @NullableDecl E expected) {
      return null;
    }
docs: Returns a {@link String} describing the difference between the {@code actual} and {@code
  expected} values, if possible, or {@code null} if not.
  
  <p>The implementation on the {@link Correspondence} base class always returns {@code null}. To
  enable diffing, use {@link #formattingDiffsUsing} (or override this method in a subclass, but
  factory methods are recommended over subclassing).
  
  <p>Assertions should only invoke this with parameters for which {@link #compare} returns {@code
  false}.
  
  <p>If this throws an exception, that implies that it is not possible to describe the diffs. An
  assertion will normally only call this method if it has established that its condition does not
  hold: good practice dictates that, if this method throws, the assertion should catch the
  exception and continue to describe the original failure as if this method had returned null,
  mentioning the failure from this method as additional information.