code: func diff(vec1 []float64, vec2 []float64) []float64 {
  	ret := make([]float64, len(vec1))
  	for i := range ret {
  		ret[i] = vec1[i] - vec2[i]
  	}
  	return ret
  }
docs: // diff returns the difference between the vec1 and vec2. This assumes the
  // dimensions have already been checked to be compatible.