Consider the following premises. p_1 -> u. p_1. u or ~s_1. Can we infer ~u from them?
 From u or ~s_1 we can infer s_1 -> u. From p_1 we can infer p_1 or s_1. Finally, from p_1 -> u, s_1 -> u, p_1 or s_1 we can infer u, which contradicts ~u. Therefore, the answer is no.