code: func (s QuadFilter) buildIterator(qs graph.QuadStore) graph.Iterator {
  	if s.Values == nil {
  		return iterator.NewNull()
  	} else if v, ok := One(s.Values); ok {
  		return qs.QuadIterator(s.Dir, v)
  	}
  	if s.Dir == quad.Any {
  		panic("direction is not set")
  	}
  	sub := s.Values.BuildIterator(qs)
  	return iterator.NewLinksTo(qs, sub, s.Dir)
  }
docs: // buildIterator is not exposed to force to use Quads and group filters together.