Consider the following premises. ~w_1. w_1 <-> s_4. p_4 <-> u. p_4 <-> s_4. Can we infer u from them? If possible, name the inference rules being used at each step.
 From w_1 <-> s_4, ~w_1 we can infer ~s_4 via biconditional elimination. From p_4 <-> s_4, ~s_4 we can infer ~p_4 via biconditional elimination. Finally, from p_4 <-> u, ~p_4 we can infer ~u via biconditional elimination, which contradicts u. Therefore, the answer is no.