Consider the following premises. s_4 <-> w_4. w_4 <-> u. s_4 <-> w. ~u. Can we infer ~w from them?
 From w_4 <-> u, ~u we can infer ~w_4. From s_4 <-> w_4, ~w_4 we can infer ~s_4. Finally, from s_4 <-> w, ~s_4 we can infer ~w. Therefore, the answer is yes.