Consider the following premises. t_3 <-> u_3. t -> r_4. ~u_3. t_3 <-> r_4. Can we infer t from them? If possible, name the inference rules being used at each step.
 From t_3 <-> u_3, ~u_3 we can infer ~t_3 via biconditional elimination. From t_3 <-> r_4, ~t_3 we can infer ~r_4 via biconditional elimination. Finally, from t -> r_4, ~r_4 we can infer ~t via modus tollens, which contradicts t. Therefore, the answer is no.