Consider the following premises. t <-> ~s. s -> t_1. ~t_1. Can we infer ~t from them? If possible, name the inference rules being used at each step.
 From s -> t_1, ~t_1 we can infer ~s via modus tollens. From t <-> ~s we can infer ~s -> t via biconditional elimination. Finally, from ~s -> t, ~s we can infer t via modus ponens, which contradicts ~t. Therefore, the answer is no.