Consider the following premises. u_4. s_3 -> ~u_4. ~s_3 -> w. Can we infer ~w from them? If possible, name the inference rules being used at each step.
 From s_3 -> ~u_4, u_4 we can infer ~s_3 via modus tollens. Finally, from ~s_3 -> w, ~s_3 we can infer w via modus ponens, which contradicts ~w. Therefore, the answer is no.