code: func (it *Materialize) Stats() graph.IteratorStats {
  	overhead := int64(2)
  	var (
  		size  int64
  		exact bool
  	)
  	if it.expectSize > 0 {
  		size, exact = it.expectSize, false
  	} else {
  		size, exact = it.Size()
  	}
  	subitStats := it.subIt.Stats()
  	return graph.IteratorStats{
  		ContainsCost: overhead * subitStats.NextCost,
  		NextCost:     overhead * subitStats.NextCost,
  		Size:         size,
  		ExactSize:    exact,
  		Next:         it.runstats.Next,
  		Contains:     it.runstats.Contains,
  	}
  }
docs: // The entire point of Materialize is to amortize the cost by
  // putting it all up front.