Consider the following premises. t <-> s_1. t. s_1 -> u. Can we infer u from them? If possible, name the inference rules being used at each step.
 From t <-> s_1 we can infer t -> s_1 via biconditional elimination. From t -> s_1, t we can infer s_1 via modus ponens. Finally, from s_1 -> u, s_1 we can infer u via modus ponens. Therefore, the answer is yes.