code: func Concat(xss ...[]float64) []float64 {
  	total := 0
  	for _, xs := range xss {
  		total += len(xs)
  	}
  	out := make([]float64, total)
  	pos := 0
  	for _, xs := range xss {
  		pos += copy(out[pos:], xs)
  	}
  	return out
  }
docs: // Concat returns the concatenation of its arguments. It does not
  // modify its inputs.