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: r -> s. t -> r. s -> t. s -> r.
r -> t can be inferred via the transitivity rule. t -> s can be inferred via the transitivity rule. s -> r can be inferred via the transitivity rule. r <-> s can be inferred via the biconditional introduction rule. s <-> r can be inferred via the biconditional introduction rule.