code: func (it *Optional) Stats() graph.IteratorStats {
  	subStats := it.subIt.Stats()
  	return graph.IteratorStats{
  		ContainsCost: subStats.ContainsCost,
  		NextCost:     int64(1 << 62),
  		// If it's empty, pretend like it's not.
  		Size:      subStats.Size + 1,
  		ExactSize: subStats.ExactSize,
  	}
  }
docs: // We're only as expensive as our subiterator. Except, we can't be nexted.