Consider the following premises. ~p_12. u -> u_2. u_2 -> u_4. p_12 <-> u_4. Can we infer u from them?
 From p_12 <-> u_4, ~p_12 we can infer ~u_4. From u_2 -> u_4, ~u_4 we can infer ~u_2. Finally, from u -> u_2, ~u_2 we can infer ~u, which contradicts u. Therefore, the answer is no.