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: w. t -> u. u -> t. ~u.
~t can be inferred via the modus tollens rule. w and ~u can be inferred via the conjunction rule. ~u and w can be inferred via the conjunction rule. t <-> u can be inferred via the biconditional introduction rule. u <-> t can be inferred via the biconditional introduction rule.