code: func (s *StreamStats) Add(x float64) {
  	s.Total += x
  	if s.Count == 0 {
  		s.Min, s.Max = x, x
  	} else {
  		if x < s.Min {
  			s.Min = x
  		}
  		if x > s.Max {
  			s.Max = x
  		}
  	}
  	s.Count++
  
  	// Update online mean, mean of squares, and variance.  Online
  	// variance based on Wikipedia's presentation ("Algorithms for
  	// calculating variance") of Knuth's formulation of Welford
  	// 1962.
  	delta := x - s.mean
  	s.mean += delta / float64(s.Count)
  	s.meanOfSquares += (x*x - s.meanOfSquares) / float64(s.Count)
  	s.vM2 += delta * (x - s.mean)
  }
docs: // Add updates s's statistics with sample value x.