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