Consider the following premises. p_17 <-> q_21. u_2 -> p_17. p <-> q_21. ~p. Can we infer u_2 from them? If possible, name the inference rules being used at each step.
 From p <-> q_21, ~p we can infer ~q_21 via biconditional elimination. From p_17 <-> q_21, ~q_21 we can infer ~p_17 via biconditional elimination. Finally, from u_2 -> p_17, ~p_17 we can infer ~u_2 via modus tollens, which contradicts u_2. Therefore, the answer is no.