code: func (it *LinksTo) Optimize() (graph.Iterator, bool) {
  	newPrimary, changed := it.primaryIt.Optimize()
  	if changed {
  		it.primaryIt = newPrimary
  		if it.primaryIt.Type() == graph.Null {
  			it.nextIt.Close()
  			return it.primaryIt, true
  		}
  	}
  	// Ask the graph.QuadStore if we can be replaced. Often times, this is a great
  	// optimization opportunity (there's a fixed iterator underneath us, for
  	// example).
  	newReplacement, hasOne := it.qs.OptimizeIterator(it)
  	if hasOne {
  		it.Close()
  		return newReplacement, true
  	}
  	return it, false
  }
docs: // Optimize the LinksTo, by replacing it if it can be.