code: func (it *Optional) TagResults(dst map[string]graph.Value) {
  	it.tags.TagResult(dst, it.Result())
  
  	if it.lastCheck == false {
  		return
  	}
  	it.subIt.TagResults(dst)
  }
docs: // If we failed the check, then the subiterator should not contribute to the result
  // set. Otherwise, go ahead and tag it.