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