Consider the following premises. p_15 -> q. ~s. ~p_15 <-> u. s <-> q. Can we infer u from them? If possible, name the inference rules being used at each step.
 From s <-> q we can infer q -> s via biconditional elimination. From p_15 -> q, q -> s we can infer p_15 -> s via transitivity. From p_15 -> s, ~s we can infer ~p_15 via modus tollens. From ~p_15 <-> u we can infer ~p_15 -> u via biconditional elimination. Finally, from ~p_15 -> u, ~p_15 we can infer u via modus ponens. Therefore, the answer is yes.