Consider the following premises. w_4 <-> t. t or ~r. r <-> u. ~w_4. Can we infer ~u from them? If possible, name the inference rules being used at each step.
 From t or ~r we can infer r -> t via rewriting. From w_4 <-> t, ~w_4 we can infer ~t via biconditional elimination. From r -> t, ~t we can infer ~r via modus tollens. Finally, from r <-> u, ~r we can infer ~u via biconditional elimination. Therefore, the answer is yes.