What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: t_4 -> r. s -> t_4. ~r. t_4 -> s.
~t_4 can be inferred via the modus tollens rule. s -> r can be inferred via the transitivity rule. s <-> t_4 can be inferred via the biconditional introduction rule. t_4 <-> s can be inferred via the biconditional introduction rule.