欲求積：xy = { (XLy + xYL − XLYL) ∪ (XRy + xYR − XRYR) | (XLy + xYR − XLYR) ∪ (XRy + xYL − XRYL) }，須知 XY = { xy | x in X 及 y in Y }, Xy = X{y} 及 xY = {x}Y。