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: s -> u. ~s. w_1 or u. u -> s.
~u can be inferred via the modus tollens rule. ~u -> w_1 can be inferred via the rewriting rule. s <-> u can be inferred via the biconditional introduction rule. u <-> s can be inferred via the biconditional introduction rule.