Consider the following premises. t <-> s_2. t -> s_4. s_4 -> p. ~p. p -> t. Can we infer s_2 from them? If possible, name the inference rules being used at each step.
 From t -> s_4, s_4 -> p we can infer t -> p via transitivity. From p -> t, t -> p we can infer p <-> t via biconditional introduction. From p <-> t, ~p we can infer ~t via biconditional elimination. Finally, from t <-> s_2, ~t we can infer ~s_2 via biconditional elimination, which contradicts s_2. Therefore, the answer is no.