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