Consider the following premises. ~p_13 <-> u. ~q_13. p_13 <-> q_13. Can we infer u from them? If possible, name the inference rules being used at each step.
 From p_13 <-> q_13, ~q_13 we can infer ~p_13 via biconditional elimination. From ~p_13 <-> u we can infer ~p_13 -> u via biconditional elimination. Finally, from ~p_13 -> u, ~p_13 we can infer u via modus ponens. Therefore, the answer is yes.