assertThat(map.entrySet()).isEmpty();
    assertTrue(map.isEmpty());
    assertThat(map.get("five")).isNull();
  }

  private static class NonNavigableSortedSet extends ForwardingSortedSet<String> {
    private final SortedSet<String> delegate = Sets.newTreeSet();

    @Override
    protected SortedSet<String> delegate() {
      return delegate;
    }
  }

    assertThat(map.entrySet()).isEmpty();
    assertTrue(map.isEmpty());
    assertThat(map.get("five")).isNull();
  }

  private static class NonNavigableSortedSet extends ForwardingSortedSet<String> {
    private final SortedSet<String> delegate = Sets.newTreeSet();

    @Override
    protected SortedSet<String> delegate() {
      return delegate;
    }
  }

        throw new UnsupportedOperationException();
      }
    };
  }

  private static <E extends @Nullable Object> SortedSet<E> removeOnlySortedSet(SortedSet<E> set) {
    return new ForwardingSortedSet<E>() {
      @Override
      protected SortedSet<E> delegate() {
        return set;
      }

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