Consider the following premises. ~u. u -> s_1. s_1 -> u. Can we infer ~s_1 or r_2 from them?
 From u -> s_1, s_1 -> u we can infer u <-> s_1. From u <-> s_1, ~u we can infer ~s_1. Finally, from ~s_1 we can infer ~s_1 or r_2. Therefore, the answer is yes.