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