Consider the following premises. t_4 <-> t. t_4 <-> t. Can we infer t <-> t_4 from them? If possible, name the inference rules being used at each step.
 From t_4 <-> t we can infer t -> t_4 via biconditional elimination. From t_4 <-> t we can infer t_4 -> t via biconditional elimination. Finally, from t -> t_4, t_4 -> t we can infer t <-> t_4 via biconditional introduction. Therefore, the answer is yes.