Consider the following premises. p_11 <-> p. r <-> q_15. p_11 -> u. p -> q_15. u -> p_11. ~r. Can we infer ~u from them? If possible, name the inference rules being used at each step.
 From r <-> q_15, ~r we can infer ~q_15 via biconditional elimination. From p_11 -> u, u -> p_11 we can infer p_11 <-> u via biconditional introduction. From p -> q_15, ~q_15 we can infer ~p via modus tollens. From p_11 <-> p, ~p we can infer ~p_11 via biconditional elimination. Finally, from p_11 <-> u, ~p_11 we can infer ~u via biconditional elimination. Therefore, the answer is yes.