code: func (d *Document) DaleChall() float64 {
  	easy := 0.0
  	for word := range d.WordFrequency {
  		// TODO: look into more efficient lookup techniques.
  		if util.StringInSlice(word, easyWords) {
  			easy++
  		}
  	}
  	hard := d.NumWords - easy
  	x := (hard / d.NumWords) * 100
  	y := (d.NumWords / d.NumSentences)
  	return 0.1579*x + 0.0496*y
  }
docs: // DaleChall computes the Dale–Chall score
  // (https://en.wikipedia.org/wiki/Dale%E2%80%93Chall_readability_formula).