Consider the following premises. p_9 -> ~p. p. p_9 <-> u. Can we infer u 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_9 -> ~p, p we can infer ~p_9 via modus tollens. Finally, from u -> p_9, ~p_9 we can infer ~u via modus tollens, which contradicts u. Therefore, the answer is no.