Consider the following premises. ~r_4. r_4 <-> t. w <-> u. w_3 <-> t. w <-> w_3. Can we infer ~u from them?
 From r_4 <-> t, ~r_4 we can infer ~t. From w_3 <-> t, ~t we can infer ~w_3. From w <-> w_3, ~w_3 we can infer ~w. Finally, from w <-> u, ~w we can infer ~u. Therefore, the answer is yes.