Consider the following premises. s_2 -> p_11. p_11 -> s_2. p_11 <-> s_2. Can we infer s_2 <-> p_11 from them?
 From p_11 -> s_2, s_2 -> p_11 we can infer p_11 <-> s_2. From p_11 <-> s_2 we can infer p_11 -> s_2. From p_11 <-> s_2 we can infer s_2 -> p_11. Finally, from s_2 -> p_11, p_11 -> s_2 we can infer s_2 <-> p_11. Therefore, the answer is yes.