{12}\left\{\frac{1}{2}\right\}{21}
R−{12}\mathbb{R}-\left\{\frac{1}{2}\right\}R−{21}
R−{13}\mathbb{R}-\left\{\frac{1}{3}\right\}R−{31}
R−{34}\mathbb{R}-\left\{\frac{3}{4}\right\}R−{43}