Consider the following premises. p_19 <-> u. p_11 <-> t. p_19 -> t. p_11 -> u. Can we infer t <-> p_11 from them? If possible, name the inference rules being used at each step.
 From p_19 <-> u we can infer u -> p_19 via biconditional elimination. From p_11 -> u, u -> p_19 we can infer p_11 -> p_19 via transitivity. From p_11 -> p_19, p_19 -> t we can infer p_11 -> t via transitivity. From p_11 <-> t we can infer t -> p_11 via biconditional elimination. Finally, from t -> p_11, p_11 -> t we can infer t <-> p_11 via biconditional introduction. Therefore, the answer is yes.