Figure P$10\mathrm{.}50$

In the answer sheet, it is connected as Ro$=$RL$+$RF$+$ro$2$ in the process of calculating Rof. But my idea is $($RL$+$ro$2||$RF$).($because Rof points middle od D$2$ and D$2\text{'})$ What is the problem and what is the right answer?