code: func (d *Document) Assess() *Assessment {
  	a := Assessment{
  		FleschKincaid: d.FleschKincaid(), ReadingEase: d.FleschReadingEase(),
  		GunningFog: d.GunningFog(), SMOG: d.SMOG(), DaleChall: d.DaleChall(),
  		AutomatedReadability: d.AutomatedReadability(), ColemanLiau: d.ColemanLiau()}
  
  	gradeScores := []float64{
  		a.FleschKincaid, a.AutomatedReadability, a.GunningFog, a.SMOG,
  		a.ColemanLiau}
  
  	mean, merr := stats.Mean(gradeScores)
  	stdDev, serr := stats.StandardDeviation(gradeScores)
  	if merr != nil || serr != nil {
  		a.MeanGradeLevel = 0.0
  		a.StdDevGradeLevel = 0.0
  	} else {
  		a.MeanGradeLevel = mean
  		a.StdDevGradeLevel = stdDev
  	}
  
  	return &a
  }
docs: // Assess returns an Assessment for the Document d.