code: func (s Sample) StdDev() float64 {
  	if len(s.Xs) == 0 || s.Weights == nil {
  		return StdDev(s.Xs)
  	}
  	// TODO(austin)
  	panic("Weighted StdDev not implemented")
  }
docs: // StdDev returns the sample standard deviation of the Sample.