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