code: func (z *Dec) Add(x, y *Dec) *Dec {
  	xx, yy := upscale(x, y)
  	z.SetScale(xx.Scale())
  	z.UnscaledBig().Add(xx.UnscaledBig(), yy.UnscaledBig())
  	return z
  }
docs: // Add sets z to the sum x+y and returns z.
  // The scale of z is the greater of the scales of x and y.