Consider the following premises. p_9 <-> u. p_4 <-> u. Can we infer p_4 -> p_9 from them? If possible, name the inference rules being used at each step.
 From p_9 <-> u we can infer u -> p_9 via biconditional elimination. From p_4 <-> u we can infer p_4 -> u via biconditional elimination. Finally, from p_4 -> u, u -> p_9 we can infer p_4 -> p_9 via transitivity. Therefore, the answer is yes.