code: func (d *Document) AutomatedReadability() float64 {
  	x := 4.71 * (d.NumCharacters / d.NumWords)
  	y := 0.5 * (d.NumWords / d.NumSentences)
  	return x + y - 21.43
  }
docs: // AutomatedReadability computes the automated readability index score
  // (https://en.wikipedia.org/wiki/Automated_readability_index).