Consider the following premises. p_4 -> p_8. u_4. p_14 -> ~u_4. p_8 -> p_14. p_8 <-> p_4. Can we infer p_4 from them?
 From p_14 -> ~u_4, u_4 we can infer ~p_14. From p_8 -> p_14, ~p_14 we can infer ~p_8. From p_8 <-> p_4 we can infer p_8 -> p_4. From p_8 -> p_4, p_4 -> p_8 we can infer p_8 <-> p_4. Finally, from p_8 <-> p_4, ~p_8 we can infer ~p_4, which contradicts p_4. Therefore, the answer is no.