Consider the following premises. s -> u_2. ~u_2. p_2 <-> u_4. p_2 -> s. Can we infer u_4 from them?
 From s -> u_2, ~u_2 we can infer ~s. From p_2 -> s, ~s we can infer ~p_2. Finally, from p_2 <-> u_4, ~p_2 we can infer ~u_4, which contradicts u_4. Therefore, the answer is no.