Consider the following premises. t_4. u. t_4 <-> p. s -> ~u. Can we infer s from them? If possible, name the inference rules being used at each step.
 From t_4 <-> p we can infer t_4 -> p via biconditional elimination. Finally, from s -> ~u, u we can infer ~s via modus tollens, which contradicts s. Therefore, the answer is no.