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_3 <-> t. u <-> t. ~r_4. u -> r_4.
~u can be inferred via the modus tollens rule. r_3 -> t can be inferred via the biconditional elimination rule. u -> t can be inferred via the biconditional elimination rule. t -> r_3 can be inferred via the biconditional elimination rule. t -> u can be inferred via the biconditional elimination rule.