Consider the following premises. p. ~u_4. u <-> u_4. Can we infer u from them? If possible, name the inference rules being used at each step.
 From u <-> u_4 we can infer u -> u_4 via biconditional elimination. Finally, from u -> u_4, ~u_4 we can infer ~u via modus tollens, which contradicts u. Therefore, the answer is no.