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