code: func GeoMean(xs []float64) float64 {
  	if len(xs) == 0 {
  		return math.NaN()
  	}
  	m := 0.0
  	for i, x := range xs {
  		if x <= 0 {
  			return math.NaN()
  		}
  		lx := math.Log(x)
  		m += (lx - m) / float64(i+1)
  	}
  	return math.Exp(m)
  }
docs: // GeoMean returns the geometric mean of xs. xs must be positive.