Consider the following premises. s <-> u_3. s -> r_3. r_3 -> s_1. s_1 -> u_3. Can we infer ~(s -> u_3) from them? If possible, name the inference rules being used at each step.
 From s -> r_3, r_3 -> s_1 we can infer s -> s_1 via transitivity. Finally, from s -> s_1, s_1 -> u_3 we can infer s -> u_3 via transitivity, which contradicts ~(s -> u_3). Therefore, the answer is no.