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_2 -> r. r -> r_2. r -> ~u. u.
~r can be inferred via the modus tollens rule. r_2 -> ~u can be inferred via the transitivity rule. r_2 <-> r can be inferred via the biconditional introduction rule. r <-> r_2 can be inferred via the biconditional introduction rule.