Consider the following premises. t <-> s. t_3 -> s. t -> t_3. Can we infer s <-> t from them? If possible, name the inference rules being used at each step.
 From t <-> s we can infer s -> t via biconditional elimination. From t -> t_3, t_3 -> s we can infer t -> s via transitivity. Finally, from s -> t, t -> s we can infer s <-> t via biconditional introduction. Therefore, the answer is yes.