Consider the following premises. s_3. q -> ~q_6. s_3 -> q_6. u <-> q. Can we infer u from them? If possible, name the inference rules being used at each step.
 From s_3 -> q_6, s_3 we can infer q_6 via modus ponens. From q -> ~q_6, q_6 we can infer ~q via modus tollens. Finally, from u <-> q, ~q we can infer ~u via biconditional elimination, which contradicts u. Therefore, the answer is no.