Consider the following premises. ~u_2. s -> u_2. u_2 -> s. Can we infer ~s from them?
 From u_2 -> s, s -> u_2 we can infer u_2 <-> s. From u_2 <-> s we can infer s -> u_2. Finally, from s -> u_2, ~u_2 we can infer ~s. Therefore, the answer is yes.