Consider the following premises. ~r_4. t -> p_6. p_6 <-> r_4. p_6 -> t. Can we infer t from them? If possible, name the inference rules being used at each step.
 From p_6 -> t, t -> p_6 we can infer p_6 <-> t via biconditional introduction. From p_6 <-> r_4, ~r_4 we can infer ~p_6 via biconditional elimination. Finally, from p_6 <-> t, ~p_6 we can infer ~t via biconditional elimination, which contradicts t. Therefore, the answer is no.