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: q_2 <-> t_4. w -> s. t_4 -> w.
t_4 -> s can be inferred via the transitivity rule. q_2 -> t_4 can be inferred via the biconditional elimination rule. t_4 -> q_2 can be inferred via the biconditional elimination rule.