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: t <-> r. r -> r_3. r_3 -> t. ~t.
~r_3 can be inferred via the modus tollens rule. r -> t can be inferred via the transitivity rule. t -> r can be inferred via the biconditional elimination rule. r -> t can be inferred via the biconditional elimination rule. ~r can be inferred via the biconditional elimination rule.