Consider the following premises. u -> q_11. q_11 <-> q. ~q_6. q_11 -> u. q -> q_6. Can we infer ~u from them? If possible, name the inference rules being used at each step.
 From u -> q_11, q_11 -> u we can infer u <-> q_11 via biconditional introduction. From q -> q_6, ~q_6 we can infer ~q via modus tollens. From q_11 <-> q, ~q we can infer ~q_11 via biconditional elimination. Finally, from u <-> q_11, ~q_11 we can infer ~u via biconditional elimination. Therefore, the answer is yes.