code: func NewLinksTo(qs graph.QuadStore, it graph.Iterator, d quad.Direction) *LinksTo {
  	return &LinksTo{
  		uid:       NextUID(),
  		qs:        qs,
  		primaryIt: it,
  		dir:       d,
  		nextIt:    &Null{},
  	}
  }
docs: // Construct a new LinksTo iterator around a direction and a subiterator of
  // nodes.