}

  @WeakOuter
  private final class ColumnKeySet extends TableSet<C> {
    @Override
    public Iterator<C> iterator() {
      return createColumnKeyIterator();
    }

    @Override
    public int size() {
      return Iterators.size(iterator());
    }

    @Override
    public boolean remove(@Nullable Object obj) {
      if (obj == null) {