Consider the following premises. s -> t. p_9 -> s. p_9 <-> t. Can we infer p_9 <-> s from them?
 From p_9 <-> t we can infer t -> p_9. From s -> t, t -> p_9 we can infer s -> p_9. Finally, from p_9 -> s, s -> p_9 we can infer p_9 <-> s. Therefore, the answer is yes.