Consider the following premises. s_2 <-> p. ~p -> u. ~s_2. u -> ~p. Can we infer u from them?
 From s_2 <-> p, ~s_2 we can infer ~p. From u -> ~p, ~p -> u we can infer u <-> ~p. From u <-> ~p we can infer ~p -> u. Finally, from ~p -> u, ~p we can infer u. Therefore, the answer is yes.