Consider the following premises. p_2 -> u. u -> u_4. p_2 <-> u_4. Can we infer p_2 -> u_4 from them? If possible, name the inference rules being used at each step.
 From p_2 <-> u_4 we can infer u_4 -> p_2 via biconditional elimination. Finally, from p_2 -> u, u -> u_4 we can infer p_2 -> u_4 via transitivity. Therefore, the answer is yes.