} else {
      return removeAllImpl(set, collection.iterator());
    }
  }

  @GwtIncompatible // NavigableSet
  static class DescendingSet<E extends @Nullable Object> extends ForwardingNavigableSet<E> {
    private final NavigableSet<E> forward;

    DescendingSet(NavigableSet<E> forward) {
      this.forward = forward;
    }

    @Override
    protected NavigableSet<E> delegate() {

      }
    };
  }

  @GwtIncompatible // NavigableSet
  private static <E extends @Nullable Object> NavigableSet<E> removeOnlyNavigableSet(
      NavigableSet<E> set) {
    return new ForwardingNavigableSet<E>() {
      @Override
      protected NavigableSet<E> delegate() {
        return set;
      }

      @Override
      public boolean add(@ParametricNullness E element) {

      }
    };
  }

  @GwtIncompatible // NavigableSet
  private static <E extends @Nullable Object> NavigableSet<E> removeOnlyNavigableSet(
      NavigableSet<E> set) {
    return new ForwardingNavigableSet<E>() {
      @Override
      protected NavigableSet<E> delegate() {
        return set;
      }

      @Override
      public boolean add(@ParametricNullness E element) {