Consider the following premises. w -> t_4. t_4 -> p_6. p_6 -> u. u -> w. Can we infer p_6 <-> t_4 from them? If possible, name the inference rules being used at each step.
 From p_6 -> u, u -> w we can infer p_6 -> w via transitivity. From p_6 -> w, w -> t_4 we can infer p_6 -> t_4 via transitivity. Finally, from p_6 -> t_4, t_4 -> p_6 we can infer p_6 <-> t_4 via biconditional introduction. Therefore, the answer is yes.