Consider the following premises. s <-> q_6. u_4 -> q_6. ~u_4. q_6 -> u_4. Can we infer ~s from them? If possible, name the inference rules being used at each step.
 From u_4 -> q_6, q_6 -> u_4 we can infer u_4 <-> q_6 via biconditional introduction. From u_4 <-> q_6, ~u_4 we can infer ~q_6 via biconditional elimination. Finally, from s <-> q_6, ~q_6 we can infer ~s via biconditional elimination. Therefore, the answer is yes.