// Guava issue 1013: AbstractSet and most JDK set implementations are
      // susceptible to quadratic removeAll performance on lists;
      // use a slightly smarter implementation here
      boolean changed = Sets.removeAllImpl((Set<V>) delegate, c);
      if (changed) {
        int newSize = delegate.size();
        totalSize += newSize - oldSize;
        removeIfEmpty();
      }
      return changed;
    }
  }

      try {
        return super.removeAll(checkNotNull(c));
      } catch (UnsupportedOperationException e) {
        // if the iterators don't support remove
        return Sets.removeAllImpl(this, c.iterator());
      }
    }

    @Override
    public boolean retainAll(Collection<?> c) {
      try {
        return super.retainAll(checkNotNull(c));