Consider the following premises. ~p_6. q_6 -> p_6. q_6 or u. p_6 -> q_6. Can we infer ~u from them? If possible, name the inference rules being used at each step.
 From p_6 -> q_6, q_6 -> p_6 we can infer p_6 <-> q_6 via biconditional introduction. From p_6 <-> q_6, ~p_6 we can infer ~q_6 via biconditional elimination. Finally, from q_6 or u, ~q_6 we can infer u via disjunctive syllogism, which contradicts ~u. Therefore, the answer is no.