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: s -> q. q -> s. r_4 -> s.
r_4 -> q can be inferred via the transitivity rule. s <-> q can be inferred via the biconditional introduction rule. q <-> s can be inferred via the biconditional introduction rule.