Consider the following premises. p_2 -> s_1. p_2 or ~u. Can we infer ~(u -> s_1) from them?
 From p_2 or ~u we can infer u -> p_2. Finally, from u -> p_2, p_2 -> s_1 we can infer u -> s_1, which contradicts ~(u -> s_1). Therefore, the answer is no.