Consider the following premises. p -> q_18. p_18 -> u. ~u. p_18 <-> q_18. Can we infer p from them? If possible, name the inference rules being used at each step.
 From p_18 <-> q_18 we can infer q_18 -> p_18 via biconditional elimination. From p_18 -> u, ~u we can infer ~p_18 via modus tollens. From q_18 -> p_18, ~p_18 we can infer ~q_18 via modus tollens. Finally, from p -> q_18, ~q_18 we can infer ~p via modus tollens, which contradicts p. Therefore, the answer is no.