code: func (it *LinksTo) Contains(ctx context.Context, val graph.Value) bool {
  	graph.ContainsLogIn(it, val)
  	it.runstats.Contains += 1
  	node := it.qs.QuadDirection(val, it.dir)
  	if it.primaryIt.Contains(ctx, node) {
  		it.result = val
  		return graph.ContainsLogOut(it, val, true)
  	}
  	it.err = it.primaryIt.Err()
  	return graph.ContainsLogOut(it, val, false)
  }
docs: // If it checks in the right direction for the subiterator, it is a valid link
  // for the LinksTo.