@NotNull
  public static FairDiffIterable fair(@NotNull DiffIterable iterable) {
    if (iterable instanceof FairDiffIterable) return (FairDiffIterable)iterable;
    FairDiffIterable wrapper = new FairDiffIterableWrapper(iterable);
    verifyFair(wrapper);
    return wrapper;
  }
@NotNull
  public static FairDiffIterable fair(@NotNull DiffIterable iterable)