Consider the following premises. p_9 <-> u_4. p_9 -> ~u. u. Can we infer ~u_4 from them? If possible, name the inference rules being used at each step.
 From p_9 <-> u_4 we can infer u_4 -> p_9 via biconditional elimination. From p_9 -> ~u, u we can infer ~p_9 via modus tollens. Finally, from u_4 -> p_9, ~p_9 we can infer ~u_4 via modus tollens. Therefore, the answer is yes.