Consider the following premises. ~s_2. q -> s_2. r. u_4 -> q. Can we infer u_4 from them? If possible, name the inference rules being used at each step.
 From u_4 -> q, q -> s_2 we can infer u_4 -> s_2 via transitivity. Finally, from u_4 -> s_2, ~s_2 we can infer ~u_4 via modus tollens, which contradicts u_4. Therefore, the answer is no.