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_1 -> r. r_1 -> t_2. r -> t_2. t_2 -> r_1.
r_1 -> t_2 can be inferred via the transitivity rule. r -> r_1 can be inferred via the transitivity rule. t_2 -> r can be inferred via the transitivity rule. r_1 <-> t_2 can be inferred via the biconditional introduction rule. t_2 <-> r_1 can be inferred via the biconditional introduction rule.