Consider the following premises. u. u_1 -> q_12. s_1 -> ~q_12. u_1. Can we infer s_1 from them?
 From u_1 -> q_12, u_1 we can infer q_12. Finally, from s_1 -> ~q_12, q_12 we can infer ~s_1, which contradicts s_1. Therefore, the answer is no.