Consider the following premises. s_1 -> r_4. p_1 -> s_1. ~r_4. p_1 <-> u. Can we infer u from them? If possible, name the inference rules being used at each step.
 From s_1 -> r_4, ~r_4 we can infer ~s_1 via modus tollens. From p_1 -> s_1, ~s_1 we can infer ~p_1 via modus tollens. Finally, from p_1 <-> u, ~p_1 we can infer ~u via biconditional elimination, which contradicts u. Therefore, the answer is no.